Op. I am so glad for you that the situation has resolved itself and that the stress this was causing you has been removed and you can look forward to a bit of peace and quiet for the birth.
This is great news and everyone has to start somewhere, But, with an eye to the future dealing with such situations, you need to think about the following comments you made and how you could improve on that to be more assertive.
Naturally I was internally overjoyed but didn't want to come across too pleased. They said that my condition had crossed their mind before asking but when DH didn't seem to think that would be an issue they cracked on
Why pretend you aren't pleased or at least relieved? You need to express your real feelings more ( you are allowed to put yourself first!) as it sounds like both your DH and ILs will keep being overbearing if you don't speak up for yourself. Also the fact that DH didn't think it was an issue and they were stupid enough to accept that is mind boggling.
I cautiously said that if I wasn't pregnant and this far along I would always help where I can ... I added that DH said he would handle things if there's really no other options.
Why are you promising to always help? Why not wait until the next bout of help is asked for and make a decision then? Otherwise you will feell obliged to help, even if its very inconvenient, because you told them you would always help. Madness. It comes across as apologising to them because you physically couldn't comply with their ridiculous inconsiderate plan that has caused you so much worry and stress.
Then you tell that them that if they can't find any other option - they've just said they can - that DH will go ahead and handle it anyway. You are actually almost saying to them if its too inconvenient they can still have the party at your house anyway? You are so lucky that they said no as you very nearly invited them to carry on. You need to check with yourself next time and keep silent if someone has just agreed to a request and not fill the conversation with extra offers. If they say they are moving the party. just say thanks. I'm relieved.
Also. You are worried about covid. You are about to give birth but your DH is going to the party anyway. Several PPs have said that he should avoid going if that is a concern. He may be fine. You may be fine. But isn't this a time where a bit of playing safe wouldn't go amiss? and he has a perfect excuse.
It doesn't sound like he is putting you or the baby first and you should def make him attend the next midwife session and ask her to tell him what she thinks.
Please look into reading about assertiveness training as it will stand you in good stead for motherhood. But its great that they have backed down and I wish you all the very best with the birth.