Re: MPLABX 5.xx, AVR questions
Each version of MPLAB X is installed independently of other versions. This allows users to continue development on older versions, while simultaneously starting new projects on newer versions. However, you should only have to install compilers, MCC, etc. once. New installations of MPLAB X should automatically find previously installed versions of compilers etc.
MPLAB X knows where all those AVR files are. You (normally) don't have to tell MPLAB X anything about how to locate them.
MCC is a separate program than MPLAB X. (Though it runs in the MPLAB X environment pretty seamlessly.) They are working hard on supporting every device, but it is not uncommon for MCC to be a bit behind on newer device support. Especially right now as traditional Microchip entities like MPLAB X and MCC are being flooded with enormous quantities of classic Atmel devices. Support is coming. ;)