Thanks Naj, always lots to think about. I have been keeping a diary for the past week - twice a day brief notes on what we did when out.
Each outing is a combo of:
1. Heelwork on slip lead - to get to the grounds but also to calm her down and break up the hunting when she starts to boil over or get distracted
2. Hunting - daily on the same rabbity ground, which she is interested in and has been worked on since little, plus a second outing somewhere new . Sometimes just letting her explore for burrows and droppings, sometimes for kibble. Usually with check cord just trailing for safety (she is liable to run up to other dogs being trained..). This is the majority of what we are doing now. Turn whistle is only occasional as since your advice she is roaming much closer and nose to the ground, and is clesely keen to stay near me so I only need to pip when she has got distracted and pulled further off without keeping an eye on me. This feels much better than the previous version...
3. A very occasional ball throw. I look back at videos where she was so driven for a thrown ball - she would pick it, and while she wouldnt readily come back, she would hold it very possessively. Now she will pick a thrown ball, run halfway back to me as I back away, or fly past, and inevitably drop the ball anywhere en route and go do something else.
You obviously have a retrieving issue that you’re working through but I see that as a separate thing and would continue sneaking in balls even if the dog tends to spit them. But that’s me.

I stopped hiding balls for her while hunting because after initially being really interested in finding them, she quickly started
a) dropping them as soon as she found them
b) running off, getting distracted and dropping them
c) running past them with a cursory nose at them sometimes
So I wondered if I was boring her because balls became less precious. I have tried hiding rabbit skin dummies for hunting and retrieving, and a rabbit ear - same problem. Not got any wings yet. But you think ball exposure in hunting will not affect ball drive in the retrieve?
You can work on the delivery with dummies. How do you reward the dog for a good delivery? That might be something to look at.

She does good deliveries at home all the time(!) as she inevitably picks up things I would rather she didn't and she recalls beautifully to the kitchen for a treat...until the past couple of days where she has started testing the boundaries by staying where she is with an excellent hold on the item, and waiting to see if I will come to her to start a chase - I have been ignoring her and letting her have it until she loses interest. On outings delivery is a different story as above, I thought because of environmental distractions (even in an empty field)...but you are suggesting maybe because of the lack of treat reward? If she did do a good delivery on outings she used to get another throw, but now much less because of wanting to not bore her.
Placeboard holds are going well for treat rewards.