Arduino pin 10 for PWM whilst SPI slave
The ATmega328p datasheet section 18.3.1 states 


  the Slave Select (SS) pin is always input. 


where using the SPI in slave with out a SS would likely have issues with sync'ing the first bit of the data. Where if that is not a problem then I would suggest that perhaps a Leonardo would solve your problem. It has 8 PWM's, none on the SPI. Whereas its SPI SS is wired to the RXLED and not to a header. Otherwise the Mega would have all what you want.