There is a tool called an "easy-out" which requires you to drill a small pilot hole and use the easy out to back out the screw by hand. The link shows you some examples of easy-outs.
the diverter knob is either stripped or the diverter itself itself may be broken turn the water supply off take the knob off a carefully remove the diverter shaft if the flapper is missing look inside the housing for the broken piece then take the name and model down to home depot or similar home store and someone will be able to match it up,. but my money is on the stripped knob
Neither one will make any difference in water pressure.
You twist the knob.
Knob will be found in a shower.
There should be a new trim plate available and new knobs. If there are no knobs that fit the stems of the faucet, there are universal ones that go on using an adapter. A square adapter goes on the stem and is held in place with a set screw and the knob fits on that and is held on with the screw in the center that goes through the knob and adapter.
Try getting the point of a utility knife blade under the edge of the screw and pry upward as you attempt to back the screw out with a screwdriver. Once the screw is out far enough, you can grasp it with pliers and unscrew it the rest of the way. If all else fails, bore through the screw with a drill bit.
Either you don't have the knob pulled all the way, or the disc in the spout is faulty.
Slide down the chrome trim at the base of the shifter knob and you will see an Allen wrench set screw. slack off that screw and the shifter knob will come off
that happens to me. i just turn the knob or move the shower head and it normally stops. if that doesn't work and it's really bothering you, call a plumber.
Turn main water supply off in your basement. Adjust or replace shower shut off knob.
There is a phillips head screw on the right side of the shift knob, as well as one on the front of the knob. I found it easiest to turn on the car then put the car in 2 and use a drill to get the screw out.
Just the knob? If there is not a knob with the correct shaped hole in the bottom, there are universal adapters that fasten to the stem coming out of the faucet and then the new knob fits on that. The adapter is just a square fitting that attaches to the stem with a set screw and the new knob fits over that and is held on with the center screw.