Congratulations on your engagement!!! If it weren’t for the rest of the story, I’d be bouncing in my chair, with happiness for you. 
But, as for the Stepmonster, etc., here’s my take, FWIW.
A) The Stepmonster already knows you’re gay, according to the OP. So, your Dad (the poor man!) probably also already knows, unless he’s in severe denial.
B) It’s time to come out to your Dad, just to be sure he knows. He sounds like he’ll be okay with it, once he gets over the initial “shock”.
C) Yes, unfortunately, you have to invite the Stepmonster. But, you don’t have to tolerate her making a scene at your wedding, should she decide to do so. Ushers can usher people out, as well as ushering them in. (Even Miss Manners would approve of removing people who are behaving inappropriately.)
D) I hereby volunteer to be an usher. (I’ll sling the bitch over my shoulder and carry her out, if I have to. Politely, of course. :D) (Miss Manners would, most likely, not approve of that method, but whatever it takes, I say.)
E) I’ll be hurt, if I’m not invited, even if I can’t be an usher. (Miss Manners would, almost certainly, not approve of my saying that. Screw her. I want to be there!)
F) Even if I’m not invited, I’d still like to send you a wedding gift, if you’ll tell me where you’re registering, or just where to send it, if a bridal registry is too traditional for you. (Back in Miss Manners’ good graces. :))