MIL and disregarding boundaries

Ask her what time, reiterate that you have plans, make sure that you're all out when she calls.

He is putting his "mentally unstable manipulative mother" ahead of his pregnant wife.

He "went off on one" when you asked him to stick to the plans made of a quiet weekend?

Why has a "mentally unstable" woman got a key to your home?

He IS a tosser, a tosser that has no compunction about upsetting his heavily pregnant wife, and you are only going to realise the full reality of that going forward.

Keep your family and friends close, you are going to need them.

You deserve better.

Infuriating as this might be, I'd hesitate to be blatantly rude or angry with her. I re-read your OP and you say:
I had responded to say this doesn't suit that we have plans Saturday and Sunday, but offered an alternative day. Her response was that she will be dropping stuff off to our house on Sunday anyway.

You told her you have plans, so there's a reasonable chance that she expects you to be out, but that she has a baby gift or something for you that she wants you to have before the birth. She might genuinely plan to just leave it at your empty house for you to come home to.

Even if the likelihood of that is small, I still wouldn't risk getting really angry with her, in case. There's also a risk you'll be adding your displaced anger with your husband.

Also husband twat. Hope you're ok.

Honestly wouldn't trust someone like that to give back the key without getting another cut.

Book some marriage counselling as with a newborn and your DH being a mummy’s boy you might need the support and help in getting him to respect your wishes and not just let your mil walk all over you both! Good luck I wish you a calm end to pregnancy and a peaceful delivery!

If you don't sort out boundaries now she'll be just turning up when she feels like it during your maternity leave.

Sorry MIL, but I am under doctors orders to rest, stay away from people and stress for the sake of the baby.
As you know stress is not good for mother or baby.
Therefore, I respectfully ask that you stay away.
Currently, we have everything we need for the birth and baby.
We look forward to introducing you to your new grandchild in the near future.

I was going to suggest a much swearyier version of what @Gymmum82 said. (She’s much nicer than I am, obvs. You should probably say her version.)
I’m pleased you lost it at DH. Unless DH KNOWS it’s a problem, it won’t BE a problem (for him - it will be YOUR problem.)

Exactly this! OP I have an unbearable MIL and this exact scenario has happened to me before.. after much worse, I am finally NC with her and it feels great

Im sorry your husband is being such a dick when you’re heavily pregnant, probably pretty stressed, and in need of support and care. His priority should be looking after you right now, even if you were being totally irrational, which you aren’t. Solidarity and hugs xx

Just say "we have decided to have some family time alone before the baby comes and really want to relax and enjoy our weekend without hosting anyone, including family. My family have also agreed to give us space and we ask that you do as well. This is quite an exciting but scary time for us ad we want to enjoy this weekend together as a couple before our baby comes and everything changes. Whatever you want to drop of DH can collect earlier or you can bring it after birth when you get to meet baby."

Genuinely though if i were you I would go out for a lovely meal or spa day because you will be spending a lot of time in the house in the next few weeks.

Mine was the same - exactly. I made the same excuses about him being an only child etc.

They are NC now and my first year as a mum was massively damaged by behaviour like this which just got worse and worse.

💖 I’m sorry. If there’s ever a time to be harsh and hormonal, it’s at 39 weeks. Your needs take priority here; I’m sorry he’s not supporting you in what was agreed. You’re really asking very little here: a weekend alone before you give birth. It’s not the moon! You’re not Mariah Carey requesting baskets of kittens.

This

I have told you we are not hosting visitors this weekend. If you turn up and let yourself in I will take your key off you and send you back home. Please don’t put yourself in that position

I'd also change the locks.

I'd fuck off out.

Be out. It's a shame you'll lose the relaxing day at home you wanted but it'll be worth it to stop her doing it again (which it probably will, if she's had a wasted journey). Act dumb when she complains. "Huh? But we told you we had plans over the weekend." You never got her message saying she was coming anyway, etc. Should do the trick. Plan something fun to do with the day.

Just lock the door and leave the key in your side...
Go to an upstairs window and tell her you are in bed.....
Your dh needs to be the grown up. He sounds like he needs a boot up the arse into manhood...

Having read your updates. I'd text her to make clear not to come and then leave your key in the door.

If you roll over on this she will likely only get worse when baby is here.

You need to stop this before baby is here, there's loads of times you'll be home but not up for visitors.

I found when I said to my OH either you need to tell her tjis or I will be sending this (insert blunt message), suddenly he was all about having a softly softly phone call from him.

Open the door naked and scream FUCK OFF

You're heavily pregnant, now is the time to have a sudden flush of extra hormones