First of all you are not overreacting at all. But when people (namely your husband) don't stand with you on something like this, it can make you doubt yourself because you're in the minority. But they are ALL wrong.
I live with a pushy family in law too so you have my understanding as well as my sympathy. Yes it is true that your husband should stand with you and not his family, the reality is that he may never do the right thing when it comes to standing up to his family.
He is likely accustomed to their overbearance all his life and possibly doesn't recognise it, or particularly want to. Unfortunately you can't change that, and if you try, you will always be seen to be the baddie.
But while that may be true, you don't have to put up with it. Just don't wait for him to change things, you're going to have to. I know when I first stood up to the in-laws it almost ended my relationship with my husband, it was horrendous. But we all came through it and have forged a new and better relationship. I don't trust them as they were extremely damaging at the time, but we are amicable and there is a begrudging respect from them now.
So, in your shoes I would tell health care professionals that you don't want ANY visitors. Your husband can't override this. He may try to pressurise you but smile sweetly and say how tired you are and it's rest you need for now, but that you'll tell him when you're really for visitors.
I couldn't tell from your post if your MIL used a key or whether the back door was unlocked. But if she has a key I'd say there's no point in changing locks as husband will need a key(!!!) and will probably succumb to pressure to hand one over - victory for the in laws - don't go there! However I don't know what your relationship is like with them and how approachable you might feel they are for you to spell it out to them that you were horrified a bit surprised when they let themselves in upstairs. Maybe make a joke about how you could have been caught in any manner of uncompromising positions (literally!!) and then explain that you anticipate after giving birth than you will need complete rest for a while.
I told my in laws that I had asked EVERYONE in my life to call before coming round to see the baby in case I was asleep. They managed to observe this for a while, then then SIL clearly decided that I must be ok by now and reverted, so had to be pulled up again reminded.
If they ignore you from the outset then your other options may be to remove yourself from the house for a while. Maybe stay with family or a friend. I once threatened to book in to a hotel (and fully intended to if I needed to) but didn't need to as miraculously my husband changed his mind. That will possibly mobilise your husband!
She is not respecting your home or your taste by moving your things around. I would either say "oh no, I put it there deliberately, thank you" or mock-horror "oyyy!!!" Or sit down and explain that you just don't move peoples thighs around in their home, that's why everyone has a home of their own. Only you will know how best your relationship with them will allow you to communicant this. But don't roll over!
You didn't have to put the wooden door letters on. You chose to, to spare her feelings. I remember feeling the same about my MIL raving about some hideous net curtains that she loved and put up in my home. I put up with them for a few months then found a 'bargain' alternative and switched them without a word. I would buy the girl letters that you like and replace the one your mil gave you, saying (if questioned) that you like to ring the changes in your home and rave about how variety is the spice of life.
You mention that you kept quiet when choosing clothes and being pushed out of the way. Careful with that, it can be seen as being a doormat and they will do it more and more if you let them. But if THEY were paying then fair play. Go back and buy the bits YOU liked with your own money.
You also say they are constantly texting and phoning. I take it they contact your husband rather than you? If so, nothing you can do about that I'm afraid. But if it's YOU they're pestering directly, just don't answer. They will start to get the message and you can use it to your advantage as a teaching session "I was resting - boy, if I need this much peace and quiet before the baby, just think how much I'll need after! I am DEFINITELY going to need people to call before visiting..."
But when all is said and done you will walk a lonely road standing up to them. But start as you mean to go on and it will pay off. Expect it to be rocky to start with though! Last thing you need on top of a new baby, you have my sympathy. Good luck!