Question #2: Can this be used at all in any way legally during the D process?
Always get legal advise from a lawyer in your state.
I believe everything is negotiable. When the negotiations fail is when the legal system kicks in. The lawyers will "negotiate" everything and then the couple settles. If you can't settle, then a judge wll rule. Then you fall onto the legal "Guidelines" or precedence. The judge can hear all claims. You can object to spouses claims. Know the rules. Play by the rules.
I had a couple "Until a judge orders me" Items on my list.
50/50 custody was one of them.
I held my boundary. I told wife I would move out of marital home AFTER a written 50/50 plan was in place. This went AGAINST my lawyers advise.
"What is best for my kids is best for me" Amor Fati Link to quotes: https://www.divorcebusting.com/forums/ubbthreads.php?ubb=showflat&Number=2879712