Well, i’m not the right person to answer this question, but i’m curious about snap proxy anyway, so i give it a try: install, setup, push… I never had going so far to answer a simple question (i hope you read this, lol).
After match: I don’t have found any reference to remove or uninstall the pushed snap, also snap-store-proxy are poorly documented and over complex like the snap it self, the fact you need online authentication for offline setup makes no sense for the sysadmin.
i have published snaps, but every time i need to do something new with snap the lack of proper documentation hold me back, then two options: ask help around the internet or brute force try and fail. In most of cases both can solved with a 3 lines paragraph.
anyway, sorry for the rant, i could not solve your problem.
Indeed, the snap remove feature is not available now in the snap store proxy. However, we are discussing our 1-2 years plans about this tool now and snap/charm removal is one of the demanding features. Unfortunately, the amount of engineering power is limited and we need to prioritize carefully.