I would loosen the tensioner before installing the belt. The problem is not so much with the adjusting bolt stripping (so long as the big 19mm bolt is unscrewed a couple/few turns), but with old tensioners losing their elasticity. If the tensioner doesn't slacken as you loosen the adjusting bolt, then your tensioner is bad.
Torque on the damper is 21 Nm at the bracket. According to the manual, torque on the fan pulley is 10Nm. If you have a torque wrench that's accurate this far down the scale, you can try it - I used one that wasn't and my pulleys ended up wobbling. I ended up tightening up as far as I could get them with a socket wrench - this might have been a little too much, but took care of the wobble.