If you use the longer screws to install the prop guards, you have to be extremely careful with those or it could damage a motor. My guess would be that part of the issue. Either you have a screw too loose on one or more motors, or too tight. There is a fine line there.
Not necessarily so, but it would be more like a motor mount, which in that case would be the lower shell. But don't panic on that just yet. Personally, I have never used prop guards on any quad I have had, even beginner ones. I see them as useless, and only adds a value of "precaution" to the flyer. But everyone has there own opinions. And yes there is a difference in the motors. Unless you have a serious motor issue I wouldn't bother with that. You would definitely know if you damaged a motor winding.
A cracked mount from the screws, possibly cantering the motor ever so slightly, which would account for vibrations and possibly a portion of the cracked mount, inhibiting the full motor rev. However, you should see an obstruction error if this were the case. It was a thought based on your description of "wobbly". You could pull the latest .dat file from the aircraft and that may possibly tell you more. At least it would confirm the rev issue with one of the motors. That is of coarse assuming that the P3s records the motor rpm's. Not sure off of the top of my head.
