I haven't found in my books the answer for effects on stability, but have found that rotors are designed in a way that retreating blade stall tend to occur before compressibility stall. So, the increased profile (question 2) drag is its first manifestation. That means more power required. As the source of power is just one (the powerplant, 1, 2 or 3 engines), the definition of profile drag/power (induced, parasite...) is just theoretical.
As the beginning of the retreating blade stall is indicated by vibration on the cyclic, than in the airframe and possibly an uncommanded pitch up (if the servos are overpowered), I think the compressibility stall may (and I'd like to be corrected) be similar, but with a pitch down tendency, due the side of the rotor the stall occurs.