After she learns the motion with a "direct lure," you'll work towards a hand signal more like this.
Yes!!! This is perfect! Great job.
Continue on gradually morphing the lure into a hand signal. Her booty is in the perfect spot now.
As a nit picky detail, I would try to do most of your reset cookies from your left hand, so that she doesn't start glancing down/forward in anticipation of food being thrown from your right side.
I love that she is comfortable basically doing a stand-stay in heel. This likely shows an understanding of "value" in heel position - why leave the good spot?
For your last "get it," can you reload the treat behind your back? Her body doesn't move, but her head wraps forward to look at your right hand, which often leads to the butt swinging out. It's better if she isn't anticipating a reward coming from that area.
Her body is perfectly lined up for the finishes without sits, but that booty swings out when she sits. Let's fix that! She has enough body awareness to be able to do it "right," but is likely just sitting how she is used to sitting on walks. We'll make a different hand signal for sitting in heel.
You can use a barrier to help with this, but you definitely don't have to use one.
Start with feeding her for standing perfectly, and then turn that into a direct lure for sitting. By "direct lure," I mean that she is munch-munch-snarfing at your hand the whole time. (If she stops much-munching, she is just going to default to sitting via swinging out.) Lure her up slightly because we want sit, but also keep her head slightly to the left.
Once she's got that down pat, we'll turn that lure into a hand signal again. That hand signal is also going to go out to the left. I'll post a video of a typical way I would signal for a booty swinger.