In your description of what's going to happen on the day of the party, you don't seem to have factored in that you might already be in labour, you might be at the hospital on the day of the party, YOU MIGHT HAVE JUST BROUGHT A NEWBORN BABY HOME FROM THE HOSPITAL.
It seems like you've come to terms with the fact that if you're still pregnant then the party is just going to go ahead. OK. But this is what you need to say to your husband and text to your in-laws.
"This party was clearly organised without my knowledge, against my wishes, and with no consideration for my well-being or our baby's.
I will not be around on the day, and all preparation and ALL cleaning will be done by you [husband and in-laws].
If I am in early labour on the day of the party, the party does not go ahead.
If my labour starts during the party, all guests leave immediately, apart from the ILs, who will stay to clean up and then leave.
If we are in hospital on the day of the party, the party does not go ahead.
If the baby has arrived by the day of the party, the party does not go ahead."
Make sure your husband knows that failure to adhere to any of the above will be a divorceable offence and you are prepared to call the police on any gate crashers if necessary.
If you do need to go to hospital before the party, lock and bolt the door.