Rom Question. S6 Edge - SM g925Z Japan Softbank.

I Have the SM G925Z Version of the galaxy s6 Edge [Japan SoftBank]

I See many Roms that say "Works with many g925 Versions !" but the Japanese Z Version is never listed anywhere.
only the F T U V P etc etc versions.

If I use TWRP 2.8.6.0 to backup my current Rom, can I try these other Roms and safely return to my Stock Rom
if cell or wifi service stop working? If I try 5.1.1 Roms, I can not return to 5.0.2 roms?

Can I use TWRP to install a 5.1.1 rom that I download, even while my current Rom is 5.0.2 ?
or can I only install Roms using Odin Flash tool ?
My ZTE Warp phone was able to install Roms from downloaded Zip files with TWRP.
But I am not 100 % sure about the process.

[ SM g925Z ] Anyone found a Rom that will work on the Japanese SM-g925Z ?

Anyone found a Rom that can run on the Softbank Japanese SM G925Z Model ? 404sc.
Ive searched all over Google, daily, None of them have officially added the g925z version.

Would it be safe to try any of the other roms posted on this site?
I have a 2 year contract with softbank, so I need a rom that wont lose access to cellular service or wifi connection.
I could live without the fingerprint scanner and the heartrate monitor if Need be.

Third question, most important maybe,
If I do a Backup with TRWP from my Rooted 5.0.2 Stock Rom, can I easy reinstall that backup with TWRP
if I try new 5.0.2 Roms, and the roms have issues? I am not completely sure, because I think you flash the rom with odin, or can you install downloaded roms with TWRP also I assume?

If I try a 5.1.1 rom, I can never return to 5.0.2 roms ? is that also true?
I dont care so much about my knox warrente IF I can swap back to my 5.0.2 Stock rom. at least I know that works.
but I dont want to get stuck with a broken Rom if I can not return easily.