in reading this...i made some thoughts...in Part 1...remove any "visual" sign of jennifer......theres still mention of her when Marty's sister sais "jennifer parker called" leave movie intact up till point where marty gets keys from biff and opens garage door..then jennifer (elisabeth shue) walks up and bout that time doc brown shows up they fly off into the sky and...cut away part 2 credits and then cut to the flying thru traffic in the air and landing.....and continue from there..that solves to me the jennifer switcheroo