Shion used a
Kalashnikov, while Kasai was carrying a
Shotgun.
first question: hard, depend of the situation
second question: as seen in the previous arc, and what we can determne in the previous episodes, the Yamainu prime objective is to protect and ensure the safety and secrecy of the institute. As result, they are forced to be discrete (so they couldn't act when the police marked teppei).
In such case, it wouldn't be that surprising if they allocated all the firearms to the assaulting team, while the back didn't have much weaponry. This might be a mistake of their part, but considering their dominating situation, they wouldn't really expect such reinforcement like Akasaka.