OP I really feel for you.
I would
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Give him a list of all the phone calls she has made to you and all the messages you've exchanged this week. Say "this communication becomes your responsibility, effective immediately".
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Give him a summary of all the hours you've spent with her over the last month. Say (if they are around) - of course, we need to also spend this time with my parents to make it fair. I will be expecting you to organise this.
When he protests at how ridiculous you're being, say - you are expecting me to do wife work. It stops now. I am going back to work, we will both be tired and grumpy, I need you to put boundaries in place with your parents. I love them but they are suffocating me. We need to agree how things will be when I go back to work, so that we share the load.
Then stop talking and listen. See what he has to say about how he expects things to be when you go back to work.
That may be a BIG eye opener. Who will do the housework, sort the childcare, take time off when the baby is sick, organise cards and presents for family birthdays, When are you both getting time to relax?
I'd think beforehand about what I wanted. Could it be you arrange a weekly/fortnightly babysitting arrangement for MIL so that you get a night out together? (assuming you're happy for her to do this?)
Can you agree a monthly Sunday lunch, and the other weekends he takes the baby over for an agreed period of time, so that you get some relaxation?
Would you like to go over to hers one evening a week so she cooks for you? Or would that be too much?
I've been doing a course on parenting, and it's got some useful strategies for dealing with situations like this.
To MIL you say - MIL, you are an amazing mum and grandma, and we are lucky to have you in our lives. We think x, y and z are really helpful /you are brilliant at. (Basically a load of positive attributes). However, we're quite different people, and although I love spending time with you, I also need my own space - especially when I go back to work. I need to make a plan for going back to work, so going forward, it works like this (see plan above). I know now that I'll have to do a lot of adjusting to manage work and looking after x, so I hope you understand that this is not pushing you away, I need to make time for everyone in my life, and I'll miss x so much I want to be able to enjoy him when I'm back.
To OH you say - you're a brilliant dad and husband, and you've worked really hard to enable me to have this vital time bringing up our son. We need to work together to find a schedule that works for you and me, and enables us to parent as a unit. I suggest x, y and z. What do you think? (The key here is to build in compromises you can be seen to be making so he feels he's had some "wins". So, for example, how about we host your parents once a month for Sunday lunch? Him - no that's not frequent enough. You - ok, then how about you take X to see them on (when it's convenient for you) whilst I do my pilates class/have a bath/catch up on sleep? Then I'll take him to my mum's/friend/baby class whilst you play the x-box/go cycling/whatever).
The other strategy was - only have 2 things you want to change in your head at any one time. So - say it's too frequent visits to yours, and the pressure to go over every weekend. You can cope with sending a daily photo. And echo back to them -
MIL - I can't believe you're limiting my time with x to this? Don't I deserve some time too?
You - I can hear that you're angry that your time with x will be limited, and you feel you deserve more time.
It does 2 things - makes people feel listened to, even if you don't change the outcome, and sometimes when they hear their words back to them they realise their demands are a bit outrageous!
Good luck with it all