So now, I feel like a 5 yr old and want to just dig my heals in and say "No, you don't get to put me in wait & see status just because your BF puts YOU in wait & see status."
You feel like a 5 year old for this not because its immature of you, but because everyone else learned by 5 years old that your friend's behavior is obnoxious.
I think its not only perfectly ok, but high time, you say to your friend "no I won't wait and see. Either you want to make plans with me, or you want to make plans with your boyfriend. Either is fine but I'm not a back-up plan. I deserve to be given priority for plans once in a while. If you can't come I'll invite someone else, but I'm sure as heck not putting my plans on hold for your boyfriend."
I mean sure, once in a while people can't commit to advance plans, and sure its reasonable for anyone to say "hey let me check on that date/time and get back to you" (and then respond in a reasonable time) but its not ok to never make advance plans and expect other people to just wait to the last minute all the time. And if she is going to choose to do that, she needs to be the one who suffers/looses out, not anyone else.
Well, there's always, "Suzie, are you really happy with being a booty call? Is that what you want from a relationship?"
I agree with both of these.
Susie's BF does not respect her and she is being a doormat. She is also pushing away friends with her lack of respect for their healthy boundaries.
If it comes to this:
...the sad possibility that she may accuse you of "being jealous" of her relationship... I've seen this sort of thing happen before.
then she is a sorry case and I would just walk away.