If you are going to D, it is important to look at it as business and make the best deal that you can. You can always be generous to XH later and take less money, offer more visitation, whatever. Right now, take care of yourself and kids as absolutely best as you can.
(1) I concur, strike quickly. STBX will only become less generous. I promise. And, he probably WILL think of alimony, especially when he has a new GF with kids who will be draining his resources. Even if he doesn't think of alimony, he WILL become more selfish. Right now, he's amenable to a pretty good deal for you. I'd snap it up. He wants the problem to go away as quickly and quietly as possible. He feels guilty and is trying to feel less guilty by being a good guy about the D. He doesn't want to put energy into anything about the old R, including working through the D. The more you haggle, push, etc., the more he'll pay attention and realize that he might want to get a better deal himself.
(2) Don't sweat even several thousand dollars. It won't matter two years from now.
(3) Don't try to control STBX's R with the kids. They already know about the relationship. Yes, share your concerns about them being exposed to overnights, but drop trying to control it. It is apt to get him riled up and to do things like go after your pension and alimony. It is apt to get GF involved in what she thinks he should go after as controlling STBX and GF's R with the kids makes the D a YOU against THEM thing rather than a YOU against HIM thing. Get OUT of his private life, and don't manage his R with the kids unless you think they are at risk for physical/emotional harm that would be actionable. That's how it will be when you are D, may as well start operating like that now.
(4) I'd ask for him to agree to keep paying at the same level of child support relative to him income that he's paying. So, maybe, state-mandated +5%, or 20% of his income, or whatever. That is, specify an amount that will grow with his income. But, include a minimum so that it doesn't fall if his income drops.
(5) Ask for child support to continue as long as the kids are in school full-time or up to age 23 max (whichever comes earlier) to help pay for college. It is far away, he'll probably be receptive. But, when they are actually in college, few kids around here seem to get support from the WAS. So, ask for it up front.
So, combining 4 and 5, maybe... Child support will be every two weeks at a minimum of (current rate). This amount will increase as income increases to be 5% above what would be mandated by the state, except that child support will continue while children are full-time students up to, but no later than, age 23 (or 24).
Really, I think the best way to protect yourself and kids is to go for a great deal now. A year from now, the deal will NOT be as nice.
----
As for your M, I'm not clear on whether you want to save it. It is still pretty early in your sitch, anything could happen. And, the sudden new R may well end just as suddenly.
But, really detaching, letting go, etc... is your best bet no matter what.
