Wow! These sure are some pretty extreme examples---to me these moms are much more demanding than the mom who simply wanted other moms to ask her first before inviting her child.
I think we all have to decide what our limits are in terms of friendships and relationships, and we all have the right to limit our contact with people who just don't mesh with us for whatever reason. When our kids are younger---toddlers, preschool, and early elementary--- it can be difficult because the kids aren't as independent and parents tend to be more involved. You're bound to run into a difficult parent or two. For me, it's really a matter of finding your balance, deciding what you can tolerate and what you won't tolerate, and being clear about that.
With "snack mom" in your example, I'd likely still invite her child, but I would tell her clearly that we would not be stopping every two hours to snack while we're at the museum. I'd inform her that we'd have lunch at ____(whatever time), and then knowing what my parameters are, it would be up to her whether or not to let her child go.
With the "constant supervision mom," I would let her know what my level of supervision is for children at that age, inform her that I would not be in the same room with them constantly, and then let her decide whether or not she allows her child to be a guest on our turf.
When our youngest was in preschool, he had a friend whose parents divorced. The mom was a friend of mine, and I'll never forget the day she TOLD me her childcare plan when she realized she had to go back to work. Guess what? She assumed that since I was a stay at home parent, I would simply watch her child a couple of days a week (ALL DAY) as her permanent daycare option. Um, no. I still had 3 other kids at home and a husband who traveled constantly for work, and I held numerous volunteer positions in the community. I told her I'd be happy to have her son come and play occasionally, but that I was not available for regular childcare.
I don't have many examples like this, probably because I have pretty firm boundaries set, and I feel comfortable making those boundaries known when necessary. So, I haven't had to do this much. Instead, I feel so blessed with the very dear, close village I share with my many friends---we all help each other whenever we can, but we also respect everyone's limits and differences.
I don't blame you or anyone for limiting contact with people who are so totally unreasonable and difficult. That's just part of life, and most of us don't have time or room in our lives for unwelcome stress.
J. F.