[chittychittybangbang]

You'd think that marking the sprockets would work but that messes up camshaft timing (torsion value). That's enough to cause rough idling, other symptoms. The TDI timing belts are designed to rotate the sprockets during tensioning except for the crank which is locked. There are videos in the timing belt article showing how far the cam sprocket will rotate. This is why you can't count the teeth and mark the belt to get accurate timing.

Do enough in the timing belt removal http://www.myturbodiesel.com/1000q/a4/vw-jetta-tdi-timing-belt-change-replacement-bew-engine.htm to loosen the belt. You don't have to remove the motor mount.

Yes, the center nut is what holds the tensioner in place.

 

[chittychittybangbang]

Just to clarify, the camshaft stays stationary because of the pin.

The cam sprocket rotates with the timing belt because the 3x 13mm bolts that attach it to the cam pulley underneath are loose. This ensures that the camshaft stays at TDC while you tension the belt. The loose sprockets ensure even tension and when you unlock the cam/crank, that they keep their relationship. If tension isn't even, after you unlock them, they'll move.

 

[chittychittybangbang]

The 3x 13mm bolts on the cam pulley should be loose while the tensioner is tightened to let the sprocket rotate independently of the pulley and camshaft. The pulley has a key on it which locks it to the camshaft. The cam pin and crank lock are in place.

After the tensioner is tightened and its nut tightened, tighten the 3 bolts and remove the crank lock and cam pin. These should all be in the timing belt writeups.