From looking at the last photo of the handle's inner "ribs" against the housing, it would appear that you've worn that area away, and that a new handle may not solve your handle-slipping "problem". You could try "roughing up" the areas where those ribs are suppose to rest against by "dimpling" some of the plastic on the housing with small punch and hammer. Just a thought. I know trying to over-tighten the screw is NOT a good idea because once the threads are stripped in the head, it's "game over". I don't know if the threads in the head are tapped directly into the head metal, OR if its a threaded insert (Like a Helicoil insert) used to make stronger tapped holes in softer materials like aluminum or plastic. IF the insert is stripped or if the threads or knurling holding the insert are stripped, you've got a much more difficult situation to resolve, and a new handle and screws will not solve your dilemma.
As far as reducing the pressure you place on the handle, that's up to you. If you've accomplished better results by doing what your doing, it would be hard to change. However, if you're a real "He Man" and have that "better-than-super-human strength", maybe it is time to re-evaluate your technique , even though it will probably take more time to complete the job using less pressure. Your personal experience with the frequency of smashed knuckles and putting up with them while trying to work through a day using your Flex will determine your decision. (Sounds like some lawyer wrote that)