I solved it.
The bone does rotate on the point where you click to select it.
The problem is I imported the Degreaser model, and its trigger hinge does not match that bone. I assumed it does and simply placed my trigger hinge in its place.
In hindsight I should probably have imported the stock flame thrower, but my worry at the beginning was that it is so old it may not even have a c_model. Degreaser being newer, I opted to import it instead. Then again, maybe even the flame thrower doesn't have a working trigger.
But MINE does now
Currently in the process of fixing another problem I found, clipping:
Removed the spare faucet that was just lying around. Managed to knock out about 1000 tris in the process, and the whole weapon looks more stream lined now IMO.
Mirrored the trigger mechanism to the right side of the weapon to get rid of the deepest bulk of the clipping issue, it also happens to show off the trigger in a very visible way.
It no longer clips when crouching.
Oh, it's gonna be gooood