Op I'm sorry this is happening.
Are the grown up children his, from a previous marriage or yours ?
Yes, he's been incredibly disloyal and dispespectful, he had his chance to pull it back and he has not.
Now is the the time for you to put your efforts into yourself, he won't listen and has destroyed your marriage.
Separate yourself from him, get an STI test, I wouldn't trust him with the viagra, he's enjoying the pick me dance from you and acting like Billy big bollocks having 2 women, his ego has just exploded.
He's a bastard, an old foolish bastard, whose throwing his marriage away for someone half his age who probably wants a green card.
Time for him to grow up, do nothing more for him, no housekeeping for this disloyal shit, take your time back and use it only for your own future security.
I really don't see why you should postpone any divorce advice at the moment, he clearly didn't need your support the past 2 years whilst she was ill.
The one thing I would think about is if he inherits anything from her, make sure that money is not diverted into another's name.
His consequenses are waiting for him, you are the one to deliver them.