About USB I have the least experience so cannot help on that. Hopefully some other kind member may know more.

The MPLABX can coexist with older 8.xx versions. Checking my Program Files (x86) folder, Microchip installs MPLABX on separate folders, so I am pretty sure you can have different versions at the same time. Have not tested that, as I had no interest so far. Just make sure PBP compiler is directed to the proper MPLABX assembler.

Ioannis