1) I belive you can get up to 42Mhz on the spi
I heard only quarter of clock speed of board can be configured for SPI. If 42MHz u mean SPI_CLOCK_DIV2 right?
For question 3, as mentioned in datasheet there are 4 Tx,Rx available physically(0,1,14-19). Im asking about 6 SPI channels? I able to find only one spi channel of pins (MOSI 75,MISO 76,SCk 74). What about other physically?
Question 4: Maximum SPI clock speed in Rpi is 16MHz so need to configure Arduino Due to 16MHz. How to do it if possible?