Correct SPI, I2C termination and EMI limiter for external bus?
At 20 MHz SPI you may have problems with signal integrity as well as EMI.
It has more to do with the rise time of the signal.
Bad signal integrity could cause SPI to not function.
Lack of ground wires in the cables could certainly increase EMI.
Both of these could be mitigated some with series resistors to slow down the edges of the SPI signals. I would consider adding extra ground wires in the cable adjacent to the SPI signals. It would be a good idea to simulate this but tools to do that are very expensive at least the ones I use. It is hard to predict what is actually going to happen.
I2C is much slower and therefore lower risk. I don't think it matters much where you place the pull-up resistors, but you probably want them on the main board so the I2C bus is not floating if the backplane is not connected.