The original actress who played Marty's GF stopped acting for a while, to take care of her mother who had cancer. I just recently saw her on something, talking about it.
Shue did a great job with the role, of course and balanced Fox's frenetic activity very well. But, at the time, I remember being disappointed when first seeing it was a different actress.
The similarities between Glover and Stoltz make them seem like a much better "genetic" fit as Father and Son, but it's clear that Stoltz was playing much more seriously and studying things (probably more realistically.)
MJF's comedic timing made those movies what they were though and while Stoltz might have also been a very good Marty, I seriously doubt they would have had the same success and I am so happy with the way they turned out!
It's always fascinating to see footage like this though, like the scenes of Luke on Tatooine. You can totally understand why they were cut, but still very entertaining to see them.