I've been following this thread and I think you are handling yourself with strength and grace.
I can't help but feel it doesnt matter which way you try and look at it, there's something very wrong with HIM not you marriage. You don't need relationship counselling HE needs to stop living in denial of the person he actually is.
Your DH runs a successful business, successful enough to employ people, provide company cars etc.
You inherited some money and put it into a high interest account for now so it can work for itself.
Your DH has issues with not having free access to this money.
Your DH isn't living in financial abuse from you, his money needs are met by whatever wage he pays himself and you actively encouraged him to treat himself to his midlife crisis car.
Whilst whizzing about in his midwife crisis car, he got SO angry you won't immediately give him access to the inheritance account and sees this as an unforgivable imbalance in your marriage he decided what he needs to do to make himself feel the Big Man or punish you is seduce his younger member of staff with his midlife crisis wagon, promises of very nice company cars and endless emotionally invested messages (of the kind he knows he used to say/send to you and made you very happy/love him)
He believes you and your selfishness with the inheritance and the arguments about it are to blame for his behaviour....
He also believes that what he did to boost his ego because of this to this lack of inheritance access doesn't matter/doesn't count/was nothing because he didn't actually LIKE this younger woman, he was just flexing his flirt muscle to see IF he could get her by saying the things he did and offering a car etc
In his mind only full sexual contact constitutes infidelity or crossing the line. Everything else is on the table, it doesnt matter what you said, what you did with the other person as long as you didn't touch and didn't reeeeally mean it. This OW was just a dartboard he was practising his aim with to prove he can still hit a bullseye.
Therefore, in his eyes, you're making a big deal out of a whole lot of nothing and he's apologised for it so you should accept that and move on. If you insist there's some talking to be done, he will accept examining your MARRIAGE (to see where he needs weren't being met and how you drove him to have to soothe his wounded hurty bits in some way) He refuses to examine himself or actually really think about how his 'target practice' has made anyone else feel, not you, not the OW and not eventually when they found out, you kids. He stole something from you all to feed his own ego.
He can't accept if what he is saying about just saying those thing to the OW but not meaning them (he didn't fancy her, just part of his target practise games), is true, it is just AS BAD as if he did mean them, because its disgusting, nasty, manipulative, user, self serving behaviour. Who can be that entitled to fuck around with other peoples feelings that way purely to blow smoke up his own arse?
Thing is, I think you suspect he did mean them, and this is how he is trying to excuse himself/minimise, but his defence is just as fucking awful, hurtful and horrible and a reflection that there's something wrong with how his brain works and he can't even see that.