Why is braking action (with Anti-Skid) best when the wheel rotates at ~85% of the actual speed?
Tire-road dynamics at speed are described by the Pacejka's "Magic Formula", so called because 


  there is no particular physical basis for the structure of the equations chosen, but they fit a wide variety of tire constructions and operating conditions. 


This empirical description was developed using regression analysis and consequently has no theoretical basis as to why it works, but it does work and is widely used. You may not like the answer that 85% is best because that is what the formula says. 

The reference gives the parameters necessary for calculating the peak longitudinal force and corresponding slip. ABS systems attempt to balance a peak slip ratio with vehicle directional control, resulting in an ideal ratio between 0.1 to 0.3. (which is 70-90% of ground speed using your terminology) 



source

I would modify your statement #2 to say that in order for the ABS to know when the tire begins to slip it has to know when the tire does not slip, which allows the determination of unbraked tire speed from which the desired slip can be calculated. I think this is one reason for pulsating brakes when ABS is active; the ongoing determination of zero slip speed as the vehicle decelerates.