Sorry but I don't think some of this rings true. MiL is terminally ill and about to have another dose of chemo. She wouldn't even offer to have the child.
Forced to move into a house you didn't like or get divorced? Doesn't sound like a good marriage to me. If you did that why are you worried about a loving grandparent looking after a baby? What is wrong with taking a baby to meet people you don't know, MiL wouldn't leave him alone with them and it is good for children to meet different people and see different homes. Do you take him to your friends?
Don't like the rest of the family either? These terrible people brought up the man you love?
Hmm sounds like a control issue to me.
Presumably you've asked everyone on Mumsnet too?
I do not know this person - AIBU?
Messy house - getting uptight
Everyone else has known for years - what did you just find out?