Very sad to see and condolences to all involved.
Of course, warbird/display flying has inherent risk, much like motorsport, and that should be recognised if you are going to participate.
I am certain that the investigation will make a good job of the findings. However, without pre-judging anything, the various video's show the basics of what happened.
For those who have not flown in that environment, here are a few points that might help understanding.
It looks like there were a few single-engined fighters and the B17 running in for a low pass over the field at around 200 feet agl and at fairly high speed. The B17 looks to have been flying fairly straight and level, the P63 appears to be in a left turn with bank and faster than the B17. As the two aircraft did collide, there would very little relative movement in the sightline between the P63 and the B17 and that lowers the likelihood of sighting the other aircraft. Additionally, the P63 seems to be manoeuvreing very close to the airfield, possibly working hard to position for compliance with the display restrictions of height, display line and crowd line. Whatever, a fast low-level turn leaves a large area under the nose of the aircraft unsighted, especially in a low-wing fighter. So, it looks likely that the B17 was not visible to the P63 pilot in that turn, right up to almost impact.
Eng