I find that anything that requires slowing down is extremely difficult to teach a cocker, it takes so much longer to teach them to stop and stay than to do anything active. But I'm thinking it could help if you train impulse control in different settings too. Imo that's the sort of training that helps all other training go easier. Just doing sessions to improve impulse control and help them to cope with frustration (frustration of having to wait/go slow). I'm sure you'll get there, didn't you post the impressive video of her sitting perfectly while you throw dummies?
Hope this doesn't sound bad, because I'm a huge fan of positive reinforcement training and I would never use aversives. But I have realised that it is possible to correct behaviour without actually punishing. I would never yank the leash or yell or anything like that. But having a command that means "wrong" or actually physically guiding the dog, allthough not really clicker theory, can be quite useful. I would always go for the clicker way first, but as you say, this is difficult stuff, so much instinct is involved.
But of course, I'm just talking general training methods, I've never trained a hunting dog. I just think this is really interesting