1. Can I play midi files with the USB to device .....or can I play only backing tracks

The CP88 has no capability to playback any files via the USB “To Device” Port; connecting a computer/laptop or tablet to the “To Host” Port, however, let’s you playback data using the Host as the player, and CP88 as the target device and audio interface. You could playback audio or MIDI using a Host application.
The CP88 receives on one MIDI channel, selectable. The CP88 can be an audio interface using the Yamaha Steinberg USB Driver. And can playback class compliant audio with appropriate Lightning-to-USB-Camera-Adapter.

2. Can I use 1 or 2 zones to control 2 additional keyboards and still use all of the CP88 (other 2 zones) sounds and mix them all together and increase the polyphony too using a total of 4 zones all together ?

All four of the Master Keyboard Zones are for external devices. The CP88’s three Parts are configured entirely separately from the four Master Keyboard Zones — which only address devices Out via MIDI

Each Live Set program features its own Master Keyboard function with four Zones. You can activate the number of Zones you need and match that with whatever you need the CP88 internal engine to do. They are setup independently... meaning your internal CP88 splits and layers are completely independent from what you configure Out via MIDI.

Thank you so much Bad Mister .So wonderful to have you here at beckon call
Gary