You will not upset the balance by changing the TRVs. It's the other valve on the radiator that governs the balance. These should have been set by the plumber on installation and should not be changed. TRVs will change the amount of water going through the rad, admittedly, but can be easily adjusted. On saying this, at least one rad should be fully open (I normally use a bathroom rad), so that the pump in the system (normally located near the boiler) isn't having too much resistance driving the water around.
I'd agree with no problem, check (using a smoke stick or similar) for draughts in the hallway. The usual culprit is the door. When ours, which is a fairly new pvc one, is just closed, it still leaves a lot of air through. It takes us to push up the handle to fully seal it.