I got around to reading the Chinese translation of volume 6 which clarified some key details for me so I feel comfortable in making a summary now. Using google translate + my weak understanding of Japanese made some details in this volume very confusing primarily because the plot of this volume is a lot more complex than past volumes. Anyways volume 6 summary:
Spoiler for spoilers obviously:
If you've read the currently translated chapters you'd know what the premise of this volume is about. Basically a trade war between Mitsugoshi company and the Trade Commerce Alliance broke out while Yukime and Sid are the third party who wants to take this opportunity to take over both businesses. Sid's primary motivation for doing so is to became "a power who controls the worlds economy from the shadows" and even though he feels bad for ruining his friends business his justification is that their company will still be running after him and Yukime's company acquires it (Sid still hasn't realized that he's essentially the owner of Mitsugoshi).
In order to carry out this scheme Sid (who turned into super agent JOHN SMITH) and Yukime will flood the market with fake bills in order to ruin the credit of both Mitsugoshi and the Trade Commerce Alliance. Sid originally only wanted to use fake bills to establish their business and earn a massive profit but Yukime interpreted his idea as using fake bills to ruin the credit of both businesses and make people lose trust in banks (basically causing a mini version of the Great Depression). Like usual Sid comes up with a "brilliant" plan without realizing how "brilliant" it actually is. Yukime does the brunt of the work by making the fake banknotes and exchanging them for gold coins and Sid acts like the bodyguard and muscle.
Meanwhile the Trade Commerce Alliance's shadow leader Gettan (月丹) who is a blind wolf beast man renowned for his combat prowess hasn't realized their schemes and is still focused on crushing Mitsugoshi who he doesn't realize is backed by Shadow Garden. He sends his three strongest subordinates to capture Mitsugoshi's leader Gamma. One of those subordinates runs into Gamma and a "fight" breaks out. I put fight in quotations since it mostly consisted of Gamma putting a stupid amount of magic into her sword, flailing wildly and tripping over herself while Gettan's subordinate is shocked that someone with so much magic is so bad in fighting. Nyu arrives and reveals that she killed the other two subordinates and in a panic the remaining subordinate tries to attack Gamma but gets killed by Gamma tripping over empty air and accidentally sliced him in half.
Mitsugoshi company realizes someone is flooding the market with fake bills and sends 666 (Rose), 665, and 664 to track down the source of the fake bills. On the way they're attacked by John Smith (Sid) who easily dispatches them with wires (he chooses to forgo using the sword in favor of wires partially to protect his identity and partially because it looks cool) but spares their life and let's them run away. Next Shadow Garden sends out Delta who instantly sniffed out that John Smith is Sid/Shadow even though Sid used perfume to try to cover his scent. Sid doesn't want Delta to report back to Alpha so he ordered Delta to hunt down the Tyrant of the "black tower" (for those who don't remember Tyrant is one of the three biggest forces in Outlaw City, he appeared in volume 5), to buy some time. Back in Shadow Garden Alpha and Gamma thought that John Smith killed Delta due to Delta's disappearance. Alpha resolves to kill John Smith.
Beta reported to Sid about several things including key business details that Sid choose to literally make notes of which was unlike Sid. Beta interpreted that as Sid getting serious. Beta tried to tell Sid about Delta's death but Sid told her that "Delta only went far away" which Beta misunderstood as Sid's "kindness" (she didn't realize he was being literal). Beta's report to Sid ends with Beta asking Sid to give her the notes he's been taking since Shadow Garden policy is that information on their documents has to be hidden using secret codes. Sid shows that his notes is already secretly coded uses five languages which causes Beta to admire him. Beta wants to learn Sid's method of secret coding so Sid decides to write her something and tells her to try to figure out what he wrote means. Beta decides to send Sid's message to Eta to decode.
Gettan also realizes that there's someone flooding the market with fake bills and realizes what a threat they are. He dismisses Mitsugoshi as the culprits because they would also be destroyed by this. He decides to turn his attention over to finding the person whose creating the fake bills because they're the more immediate threat. Private soldiers hired by the Trade Commerce Alliance find a carriage containing the fake bills but they all get killed by John Smith. John Smith is then promptly attacked by Alpha. After John Smith dodges Alpha's attacks a couple of times Alpha realizes John Smith is Shadow because the way he dodges is something only Shadow can do. Alpha is both shocked and heartbroken from this and demands to know why Shadow choose to abandon them. Sid being a cryptic Chuuni refuses to answer her which infuriates Alpha who decides to get the answer out of Sid by force. Alpha uses a attack which creates a red fog that obstructs the senses and hides her movement. She actually causes some minor injuries to Sid before Sid decides to get serious, pulls out his sword then blows the red fog away along with all the clouds in the sky. Sid and Alpha clash one more time and Sid easily overpowers her and knocks her out. With her conscious fading Alpha begs the walking away Sid to not abandon her.
With no one left to stop them Yukime and John Smith complete their plan and all that's left is to wait for the collapse of the credit system. Yukime decides to reveal to Sid why she wants to destroy the Trade Commerce Alliance. As it turns out a long time ago, in the war torn beast country she was Gettan's fiancee. Her tribe and Gettan's tribe had a alliance and they wanted to keep out of the war that was going on between the multiple tribes. In the end both their tribes were destroyed and Gettan and Yukime barely escaped. After their escape Gettan lamented to Yukime that their tribes were fools for not listening to his suggestion to take a "red pill" to gain power. To Yukime's horror he further revealed that he had killed her mother for not listening to his demands. Gettan (who is obviously deranged at this point) wanted to Yukime to take the "red pill" so they can take revenge together but Yukime obviously refused and tried to run away. Gettan attacked and slashed Yukime's back multiple times which caused Yukime's to black out. Once her conscious came back Gettan was gone and everyone she knew and loved were all dead. Since then she lived for revenge and using whatever method she can including being a prostitute and slowly collected enough wealth and power to become of the three great powers of Outlaw City. Yukime concluded her story by apologizing to John Smith for using him in her revenge. She then tells John Smith that she plans to confront and take her revenge on Gettan.
At Shadow Garden, the situation is a mess. Even though it hasn't affected them yet Mitsugoshi bank will seen be met with hordes of people who will demand to exchange banknotes for gold coins and Mitsugoshi doesn't have nearly enough gold coins to compensate. The "Seven Shadows" has decided not to tell the rest of Shadow Garden on Shadow's decision because of how it'll affect morale but Alpha's depressed and unable to operate normally which causes Gamma to overwork herself to compensate. Alpha and Gamma have a brief conversation where they try to understand what Shadow is doing and Alpha concluded that Shadow has really abandoned them because they couldn't meet his expectations. Alpha is then determined to carry out her final details and die because she promised Shadow that if Shadow order her to die she will listen without hesitation. At their darkest hour Beta barges in with Sid's message that was given to her which Eta decoded. In that message Sid reveals that he was working with Yukime and also revealed that they had exchanged collected gold coins at the place his sister was kidnapped (in the prologue) and concludes the letter with apologizing for his betrayal. Alpha and Gamma is delighted by this news since it means that if they collect the gold coins Sid and Yukime has saved up Mutsugoshi would not only survive the credit collapse but get stronger than ever. Most importantly however it means that their lord hasn't abandoned them since he was clearly telling them to collect the gold coins by putting it in a familiar place and besides, no one is dumb enough to tell you they betrayed you in a message (lol). They marveled at their lord's brilliant scheme and then headed out to collect the gold coins.
Sid goes to the place where him and Yukime's gold coins is collected only to discover it's completely gone much to his shock (it isn't made clear if he didn't realize how Shadow Garden interpreted his message or the message he wrote was complete nonsense and it just happened to form a coherent message or he didn't think Shadow Garden can decode his message). Just as he was having a mental breakdown over his plans to become "the power controlling the world's economy from the Shadows" going up in smokes, one of Yukime's subordinates comes and tells him that Yukime has been kidnapped by Gettan. Sid instantly became pissed because he thought it was Gettan who stole all his gold so he headed out to fight Gettan and take back his "most precious thing".
Go back in time a bit and Yukime has just confronted Gettan. Yukime activated the Kitsuen's tribes special ability which boasted her magic power and easily beat Gettan. Gettan revealed that back when he had slashed Yukime multiple times, Yukime had unconsciously activated the Kitsune tribes special ability and had slashed away his eyes and drove him away. Yukime considered killing Gettan but decided against it mostly due to lingering feelings. Just as Yukime turned away Gettan ate the red pill, instantly recovered and knocked Yukime out before she realized. Yukime woke up with Gettan carrying her on horseback. They have a discussion where Gettan demands to her to tell him where she kept the gold coins and then tries to get her to join him again. In the process of this conversation Gettan reveals that the Order had targeted their tribes and despite his best efforts the person the Order sent was too strong and forced him to submit to their power which is why he tried to convinced their tribes to take the "red pill". He also reveals that he'll soon join the "round table" as long as he can take down Mitsugoshi. Yukime refuses to join him once again and Gettan starts chocking her out of rage. John Smith arrives and saves her.
John Smith demands for Gettan to tell him where his "precious thing" (the gold Shadow Garden took) is. Both Gettan and Yukime thought the "precious thing" he was referring to is Yukime which caused her frozen heart to beat and her to start falling for John Smith (another romance born out of misunderstanding). John Smith stomps Gettan with his wires until Gettan decided to overdose on "red pills" which powered him up to the point where he can break his wires. Then John Smith decided to switch to hand to hand combat and continue stomping Gettan. After getting close to killing Gettan, John Smith demands Gettan to say the things he has to say (aka tell him where the gold is) and Gettan reflects on his mistakes and tells John Smith that he'll entrust "雪" (yuki or snow) to him before dying. Gettan was referring to Yukime (Yuki means snow) while John Smith thought that he meant that the gold was buried under the snow. John Smith proceeded to heal Yukime and sent her back so he can dig under the snow for gold.
(As a side note, Gettan was the only villain in this series I felt bad for. Not only did he get cucked but when he tried to entrust his former girl to the guy who cucked him, the guy who cucked him thought that he was talking about gold coins).
When Yukime returned to check on the gold coins she was shocked to see all of it was gone. Alpha appeared and explained to her Sid's "plan" and how Sid also "planned" this to save her so she can let go of her past. Furthermore because Sid order Delta to take out the Tyrant, Yukime is now the sole controller of Outlaw City. They both marveled at Sid's amazing scheming ability (lol) and agreed to work together.
In conclusion the Trading Commerce Alliance failed and Mitsugoshi was strengthened even more. The volume ends with Sid and Delta digging the snow for gold coins. Much to Sid's dismal he couldn't find any so he choose to give up and decided that he needed to avoid Alpha and let her cool off because he thought that she was pissed at him. He decided to go join Oriana's civil war due to how fun it sounds. Just as he left Delta says that she found something but Sid has already left and it isn't revealed what that something is.
Huh, that spoiler was waaaaaaaaay longer than I thought. Overall this volume was far heavier on the details and a lot of jokes/gags wouldn't be understood without reading the volume itself. I tried my best to explain all the various misunderstandings but I know it's sorta confusing. Feel free to ask if you have any questions.