If the 24v is the same as the 12v the thermostat is that alloy bit that's bolted to the block (the piece with the hose attached on it).
If you had no thermostat;
1) It wouldn't warm up
2) What fills the gap where the thermostat should go? My 'stat bolts to the block with 3 screws. What you got there if not the 'stat?
I'm afraid your head gasket is knacked.
To repair, the garage will have to skim the block to get a perfectly flat surface (sometime the head warps slightly when the gasket goes) then rebuild with a new gasket. It's a teeny amount of metal to be removed from the head, so the cambelt tensioner should be able to cope with it.
I'm worried about your mechanic...
I presume he knows what cam-belt tensioners actually do? What's he used to working on...?
They should also flush out the system and re-pressure test it to make sure the leaks have gone. Get both cylinder banks done at the same time, as overheating could have damaged the other gasket too.
I hope your problems were all caused by a failing gasket, as fixing it would return your car to good health. Water in the system would foul up the combustion and could have led to your symptoms.
I really hope that the head gasket wasn't caused by you "screwing the f*** out of it" (charming!) while there is another problem going on..
Really, beating the car up when you know it has a problem is like turfing hospital patients out of bed to see if making them run a marathon might make them better.. Chances are you'll only make the problem worse - potentially killing the patient.
Get the best mechanic you can afford. Having to continually reset ECU's should suggest that whoever is doing it is not completely competent. Aren't they interested why you keep bringing it back? You've been sticking an elastoplast on a missing limb, fella...
Dropping the engine out and fixing it properly is going to touch you just about a grand but forget that Wonderweld stuff... you have a category 1 head gasket failure! You have to own up and fix it.
Get the cambelt and tensioner done at the same time, as it'll be due one soon and you may as well get that job done while the engine is in bits anyway.
Hope you get the beast fixed..