The tensioner roller does bolt to the head, the gas strut to the block
VERY VERY GOOD SUGGESTION ABOUT THE TIMING.
Thats the only reason i'd put down for taking the head off, they'd have to get the timing out by a fair few teeth because the 20vt can still run when the timing is out by a small amount. As for having to remove the head to re-tap it? theres more than enough room if the mount is removed and the engine jacked up.
Doing a belt change on a 20VT looks a bit of a ball ache but there not too bad once all the fuel, coolant and other pain in the ass pipes are out of the way, but if the timing has been moved alot and the valves have hit then its a big job to remove the head and a very fiddly job to remove the valves and collets. I've also seen it where the gas strut blows its seals and leaks all the oil out and becomes weak then doesnt return to its origional position and when the engine is turned by hand the timing appears normal but when the engine is started and ran it knocks the timing out significantly - enough to knock the head off a valve or two - this could be whats happened in your case.