MOSFET inverter switching losses/events - 3-phase motor
accounting for the current sign change, how many of the 100 turn-ons
  imply switching losses?


Every time a FET switches on or off there will be a switching loss as the voltage and current changes, since during the transition the FET is operating in linear mode. The only exception is if there is no voltage and current to switch.

With bipolar SPWM both upper and lower FETs are turned on and off alternately to create a PWM sine wave, and this wave is applied for the entire motor rotation. Both FETs are always switching voltage and current, so every transition will cause a switching loss in both transistors. 

However all switching losses are not necessarily equal. During each half of the sine wave only one FET (eg. Q1) normally supplies power to the motor. When it is switched off the other FET (Q2) recirculates current through the motor winding. Back-emf caused by winding inductance creates a reverse voltage when Q1 switches off, which is limited to ~0.7V below ground by the body diode in Q2. 

The body diode has fairly high conduction loss, but when Q2 is turned on shortly afterwards it only has to switch 0.7V, so the Drain-Source switching loss is low (much lower than for Q1, which has to switch the full supply voltage). When Q2 is turned off again at the end of its PWM cycle the voltage will only go back to -0.7V, so again the Drain-Source switching loss is low.

If unipolar SPWM is used then PWM is only applied to each FET for one half of the sine wave, so the number of Drain-Source switching events is halved. However the other FET's body diode now has to handle all the recirculating current, which causes extra conduction losses due to the 0.7V drop.