Rudeness is awful. Some people just don't see how their actions are rude. I could say don't let her lack of social grace upset you - but that ship has sailed!

As for a Dad being on the roster the same day as you, so what? You put your name down. First. And you are about to have a baby, making it hard to help later. Perhaps he got one RDO for the month that fitted in with kinder and he thought, hey, that's a good idea. If the teacher says - one of you two has to leave, explain that you would like to stay, due to it being DD's birthday, about to have a baby etc. If the teacher lets you both stay, then there's no harm done.
If he'd gone and crossed your name off - then that would be another thing. I'm sure it's just an innocent mistake. No showdown required. He might not know your reasons for deperately wanting to help today. Just as you don't know his - ITMS.

Sounds like these last days have been a bit stressful. I hope you can relax soon.