he keeps putting pressure on me to make a decision basically to make sure he's ok. I don't know what to do, and advice is conflicting.
OK. Stop and take a breath. There are four forces at play here:
What he wants - which, frankly, is not your concern or problem He is manipulating you (and your condition I feel) because he knows if he puts enough pressure on he will get what he wants, because you have form for making impulsive decisions. All he wants is to remain in his home. Keeping the children with him will make that easier (no bedroom tax, you'll pay maintenance, he'll keep the benefits).
The advice your getting: is good but yes, conflicting. lots to think about. MIL is definitely biased in some way, whether that's due to her also being manipulated or not I don't know.
What you want: You want a clean break from your X and in many ways from the grunt-end of childcare (which I completely understand). The solution your X is proposing seems to meet those needs. But they aren't actually a package. The clean break from your X is one thing. Your long term relationship with the children is another. The way care is organised so you get some fun time is yet another.
What you want - this is where actually I feel you are being incredibly impulsive. You've been split up from your X for a few months, you're partnered up already and want to spend 5 nights a week with NewP. Too much, too soon. Think about what you want and need out of the break - not out of the current relationship.
Finally. No is a complete sentence. Use it like this.
"No XP I won't agree to that, we need to go to mediation to find the best way forward for everyone."
Repeat, repeat and repeat until it happens.