That's a very difficult position you're in, and I'm sorry for you. If you are going to say something, perhaps try to focus on how things make you feel, rather than criticising her behaviour. I'm not saying you're wrong, by the way, but people tend to react better when the conversation is about feelings of another (i.e. the impact of someone else's behaviour) rather than the behaviour itself. So, for example, you could say you find it really upsetting when your baby is away from you (rather than she shouldn't take your baby away from you). You could also say that, just as she did, you need to learn how to parent by being allowed the space to have a go. As with anything, we learn best from doing, rather than being told. You can say that you trust your hormones to guide you in doing what's right for your child - that's what they are they for, after all.
I don't think age makes any difference, when it comes to maternal instincts. I was 32 when I had my little one and I was absolutely clueless because I'd never been around babies and young children. Don't do yourself the disservice of questioning your abilities or instincts. If you are unsure of something, speak with your midwife/GP or have a little Google. There is a lot of advice around safety (particularly overheating!) that is quite different now to when our parents had us. You may well find support elsewhere to reinforce what you're doing is the advisable method (not that you need it, I hasten to add, I was thinking more to shove under your partner's mother's nose).
Do you live with her? If not firstly cut down her visits! And yes practice standing up for yourself, practice saying things when she’s not around so it might come out easier when she is “no she doesn’t need socks” “I’ll give her to you in a minute/when I’m ready”. If you don’t like her behaviour then definitely say something x
This sounds so hard. I’m in my late 30s and I can’t even speak up when my mom says stuff like this. If you think you can speak up and not get attacked, then speak up. But if you can just ignore it for a while until you’re a bit further along in post partum, wait until you feel stronger. I would also ask your partner to talk to her, it’s his mother after all. Tell her you’re grateful for her ideas but you really want to work it out on ur own etc