You ask how he lied to her?
Well I assume, as he was moving in, then didn't, he told her he was leaving you for her.
No doubt he also told her the usual drivel cheats spout - no longer loves you, only stayed for the kids, no intimacy, miserable for years, loves her, will leave you and move in...
Why on earth are either of you so desperate to keep a turd?!
As he changed his mind AFTER seeing a solicitor, it's blatantly obvious he stayed because he doesn't want to lose half his pension, and half the assets. To the point where he "attempted suicide" to make you stay.
You're deluded if you think he's there out of love - a five YEAR affair? A person who can do that to you doesn't love you. He's stayed for the lifestyle - do you wash his "little bit incontinent" clothes? That little bit will get worse...
And stop blaming her and insulting her - the way you've described yourself doesn't make you sound "better" than her - it makes it seem understandable that he probably was desperate for someone the opposite. For some fun. "Ponderous"? 😬
Instead of doing the "pick me" dance over this turd of a man, and aiming so much vitriol at the other woman I suggest you find some dignity, and take him to the cleaners via divorce.
You aren't "winning" here, over her. You think you are, but you're stuck with a lying cheat - and she's free to move on and live well. He's not a prize, he's a shackle.