That was my first thought too!
Also OP,,, as you've already noted your alarm bells going off .. I found this line concerning
" I’ve asked for months what would be expected of me financially and how would the responsibility be divvied up and he kept being evasive"
Involving your Children, asking for their input on paint colours for spare bedrooms etc.. all sounds really thoughtful on the surface, until you pair it with the above sentance. Then I think it means that you are drawn into his project and feel obligated to him without it ever being spelt out exactly what your obligations are.. and you are only finding out now... So rather than feeling guilty about this move, I would regard it as very manipulative.
It's like him deciding you'd agreed to share the cost of a car, without even knowing what that cost is.. or the make and model and whose driving. Then there's the big reveal.. "But you agreed, I know you asked again and again and I didn't tell you the details.. but look you agreed and I let you chose the colour of the car seats."
It becomes even clearer when he wails.. "but if you don't comply I'll have to get a lodger!" Translation - He'd lined you up to be his lodger! Paying 50 per cent of everything, investing in HIS asset with zero housing security, having given up your own affordable home, affordable spending plans, and independence.
And as for including costs for a housekeeper and cleaner.. What is the betting that these would be traded if you complained about the high cost of your expected contribution. "I suppose we could economise by not having a cleaner etc if you were willing to do this, then I could take that amount off your bill. " Heck by that stage he might even have you volunteering to do it, to be a helpful partner.
Another red flag.... the urgent time line... coupled with the total lack of information and assumptions of your advance total agreement. It's his timeline, not yours!
BTW... I'm not saying all this would happen or that he definitely planned it that way, just that the foundations were being laid that made it easier to happen, and when you traded in your house, you'd lose your backup.
Edited to add. But I do feel that when you pull out - there is a massive guilt trip coming your way. And all of it is based on not sharing information which is keeping you in the dark so you don't know what you are agreeing to. Conscious or not, its a worrying character trait, especially when such large sums and your whole future financial security (including finding a new job) are involved. It feels like you were expected to risk an awful lot on his say so.