Oh I expect he's spent the last year trying to get back into her bed. Probably has sometimes.
And now, he probably wants to get back with her.
Very often selfish men stray because they want more sex, or different sex, or an ego boost.
Now it's very likely that she's keen to "win" him back, so is probably providing more and better sex and generally just being very appealing. (google the pick me dance)
It's an ego boost to him that he can go back. None of the hassle of the divorce (bet he's not rushed that through, huh?)
None of the financial difficulty from it either.
So... Getting his kids back full time, losing no money in a divorce, getting plenty of sex as his (poor) wife wants to keep him...
Let me think: why WOULD he pick you over her?
No, he wouldn't.
See, if he's after excitement, you're not even that any more. He's not seeing a future with you if he's kept his kids away from you this long.
The reason he spent the day with her is because he wants to get back with her. Returns are quite common.
So no, I wouldn't accept it.
I'd slink off on my little morally bankrupt legs before I got chucked, really. Too late for you to slink off before you're cheated on though, I expect.
Them's the breaks for an OW.