I understand your concerns, ACP (Acepromazine) should not be prescribed without examination, which kind of defeats the purpose in this case. It depends on weight AND reaction (each dog differs). I don't think the vet should have left it up to you to decide how much to give her either, having just had to advise someone on another forum to go back to the vet, that she should not be giving her dog 50/100 tablets on his worst days!! TBH it's a wonder her dog was still alive.
Personally, I wouldn't give ACP to a "vet aggressive" dog, because as you so rightly point out there can be a deep seated fear instilled due to be tranquilized/immobilised but unable to react. I would however *always* muzzle the dog prior to an appointment, uncomfortable for you to watch I know, but better than giving drugs.
Perhaps diazepam would be a better option for this occasion but longer term I'd be tackling the problem by taking her to the vets, walking round the surgery etc at every opportunity just for the visit and socialisation - without anything being done to her - as this will help to ease her fears.
Hope you manage to sort it out, sounds like she is very unhappy at the vets.