For starters .... you can check your torque wrench using a weight.
Force =mass x acceleration
20nm = kg x acceleration (due to gravity 9.8m/s/s)
20 divided by 9.8 = kg at 1 metre length
2.04kg at 1 metre
if your torque wrench is 500mm long you will need twice the weight i.e 4.08kg
make sense? The weight applied with the torque wrench at the horizontal, the weight hanging down vertically! If you have access to them , try several different weights to log if the torque wrench is linear in its operation\calibration. Use the rear wheel nut as a test piece or put the torque wrench in a fixed vice on the bench.
steering head adjuster nut is 15nm - Do you have a socket to suit the adjuster nut, they are available on ebay, - gbmotorcyclespares.
Is the adjuster moving when you tighten the locknut? - if it is that's the first problem!
Tightening the top yoke nut should not affect the adjustment.
There are some proper clankies on here ( I am an electric string man really) Paul 2 Bikes will chip in I'm sure