The point would be that the in-laws don't have to be villians because they view the wedding and the activities surrounding them as a more important event that the OP's group birthday party. And the OP is not self absorbed because she views her milestone birthday, and previous plans, as something more important that her SIL's wedding.
Etiquette says that we can think whatever we want, even if it's unpleasant, but that it turns into rudeness to actually voice those negative opinions, right? For me, the ILs cross the line because according to the OP - and I'm taking her at her word because I see no reason not to - they have repeatedly voiced their unpleasant opinions so often and in such a way that the OP feels guilted and manipulated. They can have their thoughts and their views, but to unkindly verbalize them directly to the OP is coming across as trivializing her, her birthday, and her prior plans.
The problem for me, here, is not that she wants to celebrate her birthday as originally planned, but that she also wants her DH to choose her birthday. Like her, he has a choice. She doesn't have to like it, and he can do whatever he wants. He has offered a compromise that should help his family stay off her back. If her in-laws have to accept her choice, then I think she needs to accept her husband's and not villianize the inlaws because he made a reasonable choice she doesn't like.
I thought the thing that made the OP furious was that originally, throughout multiple steps of drama, the husband's initial choice was to celebrate the birthday with the wife. He had plans to celebrate with OP, the wedding changed dates, and (I thought) he still
planned to celebrate with OP. It wasn't until after a few discussions of the wedding that husband changed his mind and decided to stay at the wedding. I think from the OP's POV, she wouldn't have minded that, if it really was husband's choice. I think it looks like husband caved to whoever was going to make his life more difficult. To me, it reframes it: it's not husband's choice at all. If it was, he would have made it without his family pressuring him.
Again, this was how I interpreted things. I'm not sure exactly what, if any, pressure was exerted on DH.