I just find it really weird that just because a grandmother loves her children "passionately", one can't expect reasonable and normal behaviour from them? It is common courtesy to respect personal space and boundaries. My mother manages it and I know she adores her grandchildren.
TS123, IMHO, I would get a childminder for your son. I just think you're asking for trouble having your MIL look after your son two days a week. She will become even more possessive over him and respect your boundaries even less.
I think she wants him to want her more than you, his mother. I think you have every right to feel cheesed off and there's not a mother out there who wouldn't feel annoyed at someone who was so suffocating and needy. It certainly doesn't sound like balanced behaviour on her part.
I simply don't feel sorry or sympathy for people who try to impose their lives on others no matter how much they love their grandchildren. That is what is happening here. The grandmother has had her children and now it's TS123's time to be able to enjoy her son without having to sneak around, for heaven's sake. Why should she have to worry if she'll be able to spend quality time with her son that day?
Yes, there are grandparents who have zero interest in their grandchildren but that doesn't make the ones who think having granchildren is their opportunity to be parents again right, does it?
I just don't think you have to be quite so grateful to your MIL, TS123. She loves her grandson - that's great. She sees him 2x per week - that's loads. You're doing nothing wrong. But if you let her look after him, I just think the problem is going to get worse.
Let us know how you get on.