Although I have guessed as much that Oikura's mother was dead, I had not dwell much on the detail and definitely did not expect it to have happened this way. Having the hints/facts laid down one after another and sped up more and more was crushing, got really sad for Oikura.
In that view, I thought Hanekawa was very much on point when she questioned Ougi's thought process to arrive at this conclusion with just bare information. That just further illustrates the dark nature of Ougi, though I guess yeah... we don't know exactly what Ougi knows in the first place.
As for the last scene of Ougi and Araragi, beside the reference to "a lost child", I thought the setting and scenery itself was too familiar and sure enough with confirmation from the timeline chart...