I think things will come to a head pretty quickly now you’ve let that one drop about the birth certificate... he’s not going to let you hold that power over him for very long.
Which is kind of good in a way I guess, as you’re mental health is not faring well like this, while you’re trying to ‘grin and bear it’ whilst getting more and more anxious, unsupported and stressed.
What worries me is that you haven’t talked to a midwife (or hv now possibly?) and got their support by sharing the situation and your worries... this is a key source of support for you, and if your partner and mil are maybe going to start tantruming, I think you’ll be very pleased to have already got the midwife/hv on your side. Please phone them and ask them to visit ASAP again and make an excuse to talk to you alone?
It’s always better to have real life support, as they can also support in ways we people on the internet cannot. They can help steer a course between the rip tides of hormones and baby blues... and the hidden sharp rocks of a (rapidly!) overly possessive mil and her wet weekend idiot of a son, your dear partner! You’re trying to handle all of the above and it’s important to note that just because you may have one, it doesn’t invalidate the presence of another... ie, just because you may have raging new mummy hormones, doesn’t mean to say you don’t also have a manipulative and domineering mil! And more to the point, a weak and pathetic partner who is bullying you if you step out of line and question anything his mummy dearest does or says!
Btw, I didn’t have a jealous mil, in my case it was a vilely jealous and nasty mother... so I’m an equal opportunities ‘pushed out new mum’ :) or at least, I was, my dear little new born is a hulking 7yr old now ... and I still have mummy-son snuggles at bedtime just as lovely as when he was little (sshh, he’s supposed to be faaaar too big for that sort of soppy stuff!).
But my mother went absolutely doolally after the birth, all that ‘MY baby’ etc etc... and she demanded to be called a name that sounded well errr, very like mummy, let’s put it that way! And told all our family and friends that was her name, before I could voice any objection... which was greeted with stunned bemusement as it was so, weird.
She also took off ‘mine & ds’s fathers’ clothes each time she saw ds and redressed him in carefully chosen clothes she’d bought, even if it was just for a few hours. Late on Xmas eve she emptied ds first little Xmas stocking I’d done for him, to put in her own stocking presents for ds, leaving mine discarded at the side! At 9mths old, I know, it was for me & his father, not ds who didn’t know a thing about it, either way...). She had a strop when we had both given him hand puppets for Xmas, as ‘HERS is better than mine, and she’s got to him first, so there’s no point in even bothering with my little rag....’ sob hysterically and rushes from room trampling on her spurned offering... a lovely ikea elephant hand puppet that got years of fun from it, just as the different but equally nice rabbit puppet is given also did... the main thing being, the people who took the time to play with DS using them. Sigh. I was in the fog house ALL christmas for deliberately upsetting her with that one. Sigh again!
Anyway, hopefully you will look back and laugh... but I’m order to be able to do that, you need to create some distance right now. Emotional distance or physical distance or both, but some kind of distance is needed ASAP for you to feel safe and supported and under control again. When you are feeling under threat you can’t respond how you normally would.