Idk if this has been mentioned, but outside of the vortex, her b1,b2 is not a true 50/50 mix up. This is what I've noticed, and the reason for it is the different speed and range on her b1,b2.
b1 hits low but is slower. b2 hits high and is faster, but is much shorter range. That means outside of b2 range, b1 is batgirl's only initatior of the two. Once she's in b2 range, she can choose between b2 and b1, but b1 is slow and you can block on reflex. Which means you can block high until she goes low. All her lows are slower, her d1 is fast and looks like a low but is a mid. Also, if you have a feeling she's about to choose between the two, I think the start up is slow enough you can throw out a jab to stop her. This is harder if she's only in b1 range, of course.
However, once you are in the vortex, it is a true 50/50. after the 1,1,1 string you can only choose to block, and batgirl has plenty of time to set up the proper range for a b1/b2 mixup. Even then though, remember, b1 is slower than b2.
Don't get too stuck on that though. She has other ways of getting through. Once you're blocking too much and scared of pressure she can just quick dash in and grab. DB2 is a good and fast gap closer though I don't think there's a midscreen confirm for it. I think once people are experienced fighting her she will mostly rely on baiting out attacks in order to get in.
If I'm wrong please correct me. I am not exactly an experienced fighter.