I had this exact issue with a share horse around 7 years ago now, whom the owner said tried to kill them, when they spayed them.
I started with a small travel spray bottle, and sprayed initially away from them, and rewarded immediately afterwards with food. This gets them to associate the noise...
Notwithstanding I received advice I was very pleased with, so I'm giving my opinion from personal experience of her actual advice to me. If OP chooses not to ask advice from her that is their choice.
I can recommend Clare MacLeod. She gave me advice in 2020. Gave me a range of options for my fussy mare from company's she trusted. She is independent.
I got them on the back of my thighs many years ago when I worked with horses, so I used to tie a jumper around my waist which gave more coverage and didn't have a problem from then on. Didn't apply any creams either. These days polar trousers are great, and it sounds like you've got yourself...
I agree with most of what you are saying, but I did share a horse that had been ridden by bringing the head in, often btv. I was amazed to start with that he clearly had never been allowed to move forward into the contact, or stretch. It did take a while for him to get it, but we got there in...
I honestly don't think many medical professionals vets/doctors) are very knowledgeable on nutrition unfortunately. I do get it, they can't know everything, but it can certainly be part of a puzzle that could have been fixed much sooner if they had.
This is why I really support BTRC because they are assessed by their vet before being rehabbed as necessary, and then matched to a suitable adoptee, following their ridden assessment.
I think it's good to remember that when they are travelling they are not standing still, they are having to use muscles to keep their balance, and will also move their feet to accommodate that too, so to me it makes sense that they don't swell when travelling, only after standing in the stable...
Could you go HC, and if she knocks a pole, then treat as a schooling session to get her back on her hocks, even if just the next fence clear and then leave the ring, to try and break the cycle in competition.