"Let's keep thinking it through" = "I need to tell my lawyer you didn't bite".
He has already taken legal advice and I would bet he's been preparing for this for a long time, probably with the intention of divorcing you when DD turns 18. And he is absolutely planning to shaft you. The affair and its discovery may have knocked him a little bit off-course but he's still confident he has the upper hand .
Anyway, let's suppose for a moment that OW is not a wanton home-wrecker but has been told by your H that you have been living "together but separately" for a long time with the aim of splitting for when DD goes to college. To that end, he is supporting you in training for a new career.
Since his friendship with OW has coincided with his gradual withdrawal from the marriage I think it's very likely he has been feeding her this line for a long time - not necessarily with the objective of shagging her, more trying on his new persona for size.
And she's bought it all, because initially they were just friends, and then he was a shoulder to cry on when her engagement ended (SO predatory - what a creep!), and then he pursued her, and now he's open about them being together - if fact he's actually joining her on holiday next week!
If that's the case, he NEEDS to maintain the fiction that the divorce is really just formalising arrangements that have been in place for some time - after all, he lives in the annexe! And what with your course and DD needing to get used to the idea, there's no reason to rush especially if it lets him hold onto more money. Clearly the truth will come out eventually but by then she'll be invested in the relationship,with him.
Whereas if she suddenly and shockingly finds herself named as co-respondent in a divorce petition brought by his wife she will be alerted to the fact that he's a conniving adulterer who has lied TO HER without conscience as well as to his spouse. And you have proof of adultery from both of them (the card and the recording) so I don't see how they could challenge it.
Be a real shame if he lost his hot new younger woman as well as 60 percent of his assets, wouldn't it?