It all seems very predictable, OP. Relationships can be messy. Many of us make mistakes at various points - thinking the situation is one way when it's really more complex, and closing our eyes to truths we'd rather not see. Reading into things like a proposal what we want to see rather what they really indicate.
It sounds as though he was coerced into your marriage to some extent. I don't mean to imply you did this deliberately, but you pushed for commitment early on when he was losing everything and you were a life line. He took the life line and neither of you seem to have stopped to consider how circumstances might have made that seem like no choice.
Sarah isn't going to leave you alone. She likes your DH. She may fancy him or love him in a romantic way, she may not. But she clearly isn't prepared to stop talking to him because you don't like it. And why should she? She doesn't owe you anything and she has a decades long friendship with your DH.
And your DH is clearly still holding some sort of flame for her and isn't prepared to cut contact. It also sounds like he's not the best in terms of being noble or honest about things (the affair with you while he was married AND as your boss), the proposal that looks very much like a bid for a secure home, the not telling you about Sarah when he's clearly still in contact (though, tbf, you sound a bit controlling about that - trust him or don't, don't tell him who he's allowed to talk to).
So yes, I think leaving this marriage might well be the best option for you. But I suggest marriage counseling (and probably individual counseling for you too) before you make any hasty decisions. You are reading a lot of things that may or may not be valid into actions that could be innocent and seem quite focused on a woman who has, as far as we know, just turned up to the funerals of a couple of people she knew well and texted your DH with friendship level texts. But despite that, your posts give the impression you think your DH isn't keen on showing you that you are his priority, and that's not good in a relationship.