you can deffinately fly any of the cessna family (with its excellent trimmer) with thumb and fore finger.... take it from a piper pilot!
Holding it too tight or ridgid will transmit stress through the wrist and up the arm... i had an instructor who gave me an excellent tip.. wedge your elbow into the door rest and hold your arm there, you will then control the yoke better and won't need so much wrist strength.
As for the RPM changing..engine being weak.... check embarrasing posts by me a few years ago where i could not control the rpm in the tommie i was flying... the answer for me was attitude control.. i found if i maitained a constant attitude (vsi was less than +/- 2) my RPM magically stayed where i'd set it.