Okay after a whole year of trying almost everything to find out why my battery is flat within 2 hours of shutting down the car, and having to disconnect the battery all the time, I found the solution by accident.
I've taken the car to all manner of auto electricians/MB technicians who couldn't figure out what the problem was. Just today I asked my tech to replace my battery with a new MB one for no reason other than I wanted a battery that filled the battery space properly, and the problem was sorted. Just like that.
Everyone I went to said the old battery was fine, strong, holding charge well and definitely not the problem. This was clearly wrong and I think there may have been some kind of intermittent problem with it. Also, the car starts up more quickly and turns over more aggressively than ever before
Moral: If all else fails, buy a new battery.