1) MJ's parents being divorced (and this making MJ apprehensive about marriage) was revealed in ASM #194 during the Wolfman run and MJ's sister Gayle was revealed via a daydream in ASM #246 (I believe) during Stern's run. Obviously these elements were important in MJ's origin from ASM #259.
2) I believe I once read a statement from Mr. Frenz that during his and Mr. DeFalco's run they had plans for Peter and MJ to get engaged but for MJ to jilt Peter on the day of the wedding. I apologize if I'm getting any details wrong there.
Yes, our plan was that MJ would leave Peter at the altar.
My question is where those plans around when you guys did ASM #259, like was that issue written in any way with the intention of eventually leading into that potential story?
I'm afraid I don't remember when we came up with the leaving-at-altar idea.
3) Finally I've always noticed a really interesting parallel created by MJ's backstory between her and Peter.
Was that an intentional aspect of the story you guys built into the issue?
It was. In many ways, MJ spent her life avoiding responsibility in contrast to Peter who learned from his mistake and accept responsibility.
Hope that helps!
Thanks for being there.