Hows everyone doing?
Well I will be a new boxer owner in about 2 weeks and I did alot of research on ACE especially ( I was thinking of cropping my pups ears). Hopefully this link will help you understand more why ACE should not be used on boxers.
http://iupucbio2.iupui.edu/bml/cropdock.html
Sorry I couldn't hyperlink; just copy and paste
In reference to the vet situation I would find another vet. Especially now that you might doubt your vet due to what happen, I think that you should find a vet that is very informative on the Boxer breed and that loves them just as if they were their own children.
You will need to put alot of time into finding a new vet but it is well worth it; knowing that everytime you take your pup there they will be getting the best possible treatment.
Remeber good things come to those that are patient.
Good luck and hope every thing goes well.