Two things occur to me;
1 You are in Nl where it rains, the air is salty, you are not afraid of the rain and the stem bolts are exposed to the weather. In these conditions, in spite of all the theoretical arguements about accuracy of torque recommendations, greasing the threads to protect them would make sense. My choice would be low friction stuff like MoS2 (as sold/supplied for automotive cv joints) or Brucey's SFG but the choice is yours.
2 It only takes a little bit of loose metal/dirt on the thread going into aluminium to make the threads pick up a bit and a couple of goes later the thread is damaged enough to no longer accept being tightened correctly. Degreasing the threads and cleaning them with a soft wire brush (like the little brass brushes once used on spark plugs) before regreasing the bolt (and degreasing/cleaning the threads in the stem - not easy I know) would probably save the stem a bit.
3 (Ok I only said two!) It is highly likely that the maker did not intend his product to be undone and done up as frequently as you have to. I don't suppose there is a lot of difference between makers in this respect and they are all probably using a flavour of 6000 series alloy. It may be that you simply have to accept on security grounds that there is a limit to the number of times you can repeat the undoing-retightening cycle and that stems have a finite life before being recycled. It's for you to decide how many cycles you can do but keeping a count would be a good idea (and keeping a replacement stem to hand!)
That torque wrench needs testing with a known weight. Something is clearly wrong.
My rule of thumb method for this kind of bolt is:
1. Tighten with the "wrong" end of the allen key, the one that gives least leverage
2. Then half a turn with the "right" side
I have never stripped a thread on a bike, at least not that I can remember.
The rule of thumb (worked out by the USAnians) for construction equipment was "snug + two flats" but that was on steel components. Yours would come out at + three flats which might allow for stretch in aluminium or might be a touch too far. (It is of course a far cry from the snug + 2 turns for the sidebolts on a big hydraulic breaker!).
The other alternative (as my dad assured me was used by RR on aluminium motors in things like Princesses) was a short series spanner and "as tight as you can!"
