Ugh. I totally feel your pain. My ex's mum was/is like this. Within DAYS of ex and me telling her I was pregnant, she was phoning and telling us about this "Rolls Royce" of prams that she'd decided we NEEDED. We were nervous about buying anything in the first trimester, plus had NO IDEA what pram (if any) we wanted, so asked her not to buy it. We asked several times: PLEASE don't spend your money on something we're not sure about. She insisted. She spent hundreds on it. I used slings for the first year, and she made bitchy comments whenever she could. It was really hard because I felt so guilty (although looking back, I think we did everything we could to dissuade her).
I know a lot is said about these controlling MILs/Mothers being "well meaning" etc etc. I probably sound like a right bitter, old cow, but I really feel that they're being selfish. It's selfish to impose yourself on your daughter/daughter-in-law, and turn up uninvited. It's also pretty self-centred to "assume" you'll be very involved to the extent of baby staying with you! Seriously, women who do this, do NOT respect the mother-infant bond, or other people's boundaries. I have also always thought that the rampant "buying" that often happens is really the MIL/Mother essentially trying to muscle in, in a rather insecure way, as if they can't bear the fact that it's not about them.
Don't get me wrong, grandparents are really important, and of course they're excited, but behaving like this is just inappropriate. It also gets the new family dynamic that is developing, off to really bad start!
The reason it feels so controlling when they do it, is because IT IS. It feels like someone is trying to muscle-in on what is actually a really personal and special experience. Sometimes you might wonder WHO is actually going to be giving birth to the baby!!! It feels like your rite of passage is being encroached upon. With ex's mum, I always ended up thinking: "You had your chance love, this is MY EXPERIENCE". Why can't they express their grandmotherly excitement in more emotionally aware ways???
My own mum was/is great (luckily). With DS and with this baby, she's super excited, but she's more than aware that she's the grandmother. We went shopping together, and spent a lot of time chatting about baby care ideas and probably will this time too. It was great, even thought she didn't like some of my ideas (co-sleeping, breastfeeding long term), I never felt like she would try to change my mind. I also really appreciated when my parents gave us some money and said "have fun getting some lovely stuff for your baby". It was very generous in more ways than one.
My DH's mum has been great during this pregnancy so far. She's really excited about this new baby on the way, but has made NO assumptions about her role and how involved she will be. I guess it's because she knows we get on, and have a good relationship. She has not reason to "stake her claim", because I will be more than happy to fully involve her in our lives. She's been upfront though and has said: "we're here, and we will support you whenever you need us" i.e: we choose. Again, really warm, and generous. Why can't more MILs be like this!?