How do the high voltage lines disconnect from the ac?

With the condition of that plastic tab, I would say you will have to force something. If it makes you feel any better, I recently had to break the plastic tab off the plug that connects the inverter pump to its power source, because it wouldn't budge otherwise. So far the plug has stayed connected after several thousand more miles.

At least I was dealing with a 12V connection rather than a HV one. Disconnecting the orange safety plug (if that's what you meant by maintenance switch) was a good start, but you should still carefully probe the connection with a grounded wire while wearing electrical gloves before attempting to force it open with a screwdriver, I believe. If it was my car, I would probably try to break the rest of the plastic tab off first and then pry the connection open with a screwdriver. Hopefully, the connection could be taped over with electrical tape to keep it secure when you are finished working with it.