I'm so sorry for your loss.
You can report this vet to the state veterinary board for investigation. If the board agrees with you, the vet will get a strike against their license (so owners will know there was a problem) and possibly a fine.
It sounds like they gave they had premeded with a sedative/tranquilizer. Normal, but I wonder what drug they used. He probably reacted to that (before under the gas (although I wonder if they even use gas there with this kind of buiness going on). They may have just brought him back and then masked him down or something. Completely and totally unacceptable. They should have brought him back and watched or sent him straight to a specialty hospital and they should have recommended that all further procedures be done in a specialty hospital with an anesthesiologist and cardiologist available.
I don't buy that they actually thought it was a good idea to go on. Also, if they don't have warming equipment, at a minimum they sure as heck should have hot water bottles there or a SnuggleSafe.
Heart failure and electric shock (both of which could apply here) can cause pulmonary edema. Oxygen and drugs should have been given.