Issues with nursery

What is creepy about Mums and Babies? Isn't that where the child pretend plays with dolls to care for them?
Does your little boy happily talk about the other nursery staff? I would be concerned if he happily talks about the other staff and avoids talking about her. He may just be emotional as its a big change being in a new room and maybe the staff ratios are different so he actually sees her much less and that could be what's upsetting him?

I would start by asking them to move him down to the appropriate room for his age! Pre school will have some kids in there who are almost 5, that's too much of an age gap to be honest. Also, I appreciate he may have a strong bond with someone but that's not a good enough reason to put him somewhere that he's not developmentally ready for?

If you're not 100% I would move the nursery. Childcare is so important for you and your child and expensive so if you don't think it's right, find somewhere else. The fact the nursery let him move with her so young into a room that isn't age appropriate doesn't seem right. Some places are good for a certain period but not forever, and some nursery rooms are better than others.

My own experience with a nursery I wasn't happy with I moved to a childminder that I was happy with for about 18 months then I felt like my son would be better off with more structure, and he was showing signs he wasn't happy and put him into a new nursery which he loves.

Always trust your intuition when it comes to your kid! It doesn't have to be 100% accurate or make sense to anyone else or have enough evidence to get a conviction. It's enough to say, I know my kid, and I'm getting a weird feeling, so I'm putting my foot all the way down.