Hi, this is an italian forum, so please we need to speak in italian.
i try to answer to some of your questions:
- System partition is fully occupied by rom and system apps, so is necessary to do first root and use an app like system app remover to remove system apps (chrome, some google apps like maps, drive, photo, drive) to have free space.
With this supersu.zip you can have root and see app icon:
SuperSU 2.76 ZIP (via recovery TWP)
and this is busybox: Busybox 1.24.2 all zip (via recovery TWRP)
If you need more free space on system app, try to flash Marsapa V4 rom via ctr recovery and check:
https://mega.nz/#!10p0ECQZ!ghxnkUJTG...y_F8ZFfVL8tdus
if you need more help, please use translator and ask wahetever you need in Italian.