See below
Can my FT-857 be controlled with a separated control panel from one RemoteRig and with CAT control (HRD) from another RemoteRig box or a microRRC at different times? That is a single RemoteRig at the radio end and several RemoteRigs/microRRC's at the control end.
I don't know, you can test it your self. If the FT-857 can be controlled via CAT even if the Control panel is removed it will work. TS-480 work without control panel but most rigs does not.
Can RRC's with hardware modification for FT-857 be used with other radios after the modification? That is does the hardware modification spoil normal operation when later configured for other radios?
No it's just the speaker out via the TTL which is removed, but it's easy to modify it back to orginal, just replace the trace and 0R resistor with a wire.
73 de mike