Yes, I do know this situation and how serious it is and how complicated it is.
You know you need to make a break in the long term and that break needs to be very VERY well planned.
They won't let go. Your partner will not see reason, that would already have happened if he could.
In the short term you need to manage the situation on a daily basis and that requires a different strategy altogether which is what you are trying to figure out here and need help with.
To start the long term strategy - make a very covert call to Women's Aid and ask for their help.
In the short term, sadly I don't think your partner is ever gonna be on your team or see your POV - he's too enmeshed.
You need to make yourself as boring or invisible to them as possible.
Sounds weird but don't arouse suspicion, don't let on that anything's out of the ordinary or that you're uncomfortable with them, don't let them suspect that you're planning anything.
In this period you can't let your child out of your sight, even if it means extended BF - and I know how extreme that sounds!
But mainly get the professional help Women's Aid can give and very very good legal advice.