We do pivots for the directed retrieve similarly to how you describe, except that the dog is not looking at a fall - the gloves are positioned behind the dog/handler and you pivot to face the one the judge tells you to retrieve. Since the gloves are only 20-30 feet away and the dog can see all of them at once it is crucial that they stay in perfect heel position, not just because they can be scored for being out of position but because being out of position makes it that much easier for them to accidentally “lock on” to the wrong glove.
The first thing you need to do is work on the behavior outside of the context of a retrieve of any sort. Pivots are an ideal thing to practice on rainy days as you can do them in the kitchen or living room. Reward perfect heel position and correct it if it’s not. The dog needs to learn how to move with you - it also helps if you practice your footwork so that you are both cueing the dog properly and not accidentally pushing them out of position. We practice by standing on a paper plate until we can pivot 360 degrees without stepping off the plate. Once you move to doing it a field situation, I would start with bumpers as they should be slightly less exciting to the dog. Work them on a leash, and do NOT release the dog until it has done the pivot correctly. You can use a length of cord slipped through a collar so that once you are pleased with the pivot, you don’t have to fuss with unleashing the dog - just let go of one end and send them. The reward is the retrieve and it is not allowed to occur until you have gotten the desired behavior from the dog.
We do a lot of attention work, and I think that would help in this case as well. Teach the dog a cue to look at you (I use “watch me” but anything will do). We reward the watch by spitting food at the dog so they are rewarded from our face for looking at it. Once the dog is solid on that you can practice having them mark the fall and then look up at you for further direction. So the sequence would go - dog marks, looks at you, pivots, and then marks again. At first you would probably need to give the cues verbally but once the dog gets used to the sequence, they shouldn’t need reminders. You probably wouldn’t even need them to really stare at your face the way we like, but you do need them to be attentive enough to notice that you are changing position.
I don’t know that you will ever get her to sit in a relaxed manner, but you can certainly enforce a stay. Stay means stay EXACTLY how I put you - do not creep forward. Keep her on a leash and if she creeps forward, then no retrieve (have someone else get it or tie her up and get it yourself). She needs to understand that the ONLY way she gets that bird is to stay in the proper position. Anytime she creeps forward and is then allowed to do the retrieve she has then been rewarded for it. It sounds like you are making allowances for her because you like her drive, but it’s those high-drive dogs that you really need to exert control over. We joke in class about the ones we need to motivate vs. the ones we need to control, LOL. Don’t worry - if she is as high drive as you say, you won’t lose any of that drive by enforcing self-control.
Hope some of this is helpful at least.