I'm still in the process of looking over this thread, but I wanted to go ahead and clear up some confusion about the SS pin.
The main purpose of the SS (Slave Select) pin is as an input to allow an SPI slave to be selected. So, as the datasheet says, if the ATmega32U4 is configured as an SPI slave, the SS pin (PB0/Arduino pin 17) is always an input and determines whether the SPI interface on the ATmega32U4 is active.
If the ATmega32U4 is configured as an SPI master, there are two possibilities: the SS pin can be configured as an output or as an input. In the first case, as quoted from the datasheet:
The Arduino SPI library sets SS to be an output, and in this case, the SS pin can be, but is not required to be, used to drive the SS/CS pin of the slave. You can find many Arduino programs that use a pin other than the AVR's SS pin to control the slave device's SS/CS. So TSG is correct: the microcontroller pin that the nRF24L01 CSN pin is connected to can be user-defined; as long as the pin specified in the code matches the pin CSN is actually connected to, it should work. (Furthermore, as TSG pointed out, the microcontroller's hardware SS pin is not exposed on the A-Star 32U4 Micro.)
The text from the datasheet that Jim quoted earlier ("If SS is configured as an input...") applies only if the SS pin is configured as an input while the ATmega32U4 is configured as a master. The reason for doing this is so that you can make a multi-master SPI bus with a mechanism to avoid contention between different masters.
I will read through this thread again more closely and see if I can suggest anything else toward getting the nRF24L01 working with the A-Star.