Overtake
Using just the first images I considered that the P-63 had to have a high overtake V over the B-17, the images looked like that... and that would have placed the B-17 higher in the screen of the P-63, which would have supported the search by the pilot being towards the further forward P-51. 2's up suggested otherwise, and that needed a lower rate of closure, and the later aft aspect video shows that to have been the case. Once established in the turn the P-63 would not have had sight of the B-17 until around 0.2 of a second before impact, the accident was unavoidable once the turn had been commenced. The P-63's relative velocity to the B-17 was not a high overtake rate at all.
humble apologies.
RIP guys, sad day all round