a) Using external USB midi controller to play or remote control the Anyma Phi is the reason why we integrated a USB Host plug. Only thing to know is that we didn't had the chance to check every MIDI controller with the Anyma Phi but any class compliant USB MIDI controller should work.
We tested the Korg Nanostudio which is a superset of Korg Nanokey2.

b) MPE compatibility
As a monophonic synth, the Anyma Phi is not really MPE compatible but can be setup to behave as one MPE voice

c) yes adaptation of all the patch is possible to use the Piezo as a note trigger instead of external MIDI device

Hope this make things clear to you