Enable /system write in TWRP - Systemless root Moto X Play (XT1562), SuperSU 2.78 (systemless root), TWRP 3.0.2-r3


When I flashed TWRP, I chose the option of not writing to system to enable OS update (and also made a Nandroid just after flashing it as a backup)
Now, I made some minor changes to system audio files, without dwelling on consequences -  won't be able to update OS ( update expected this quarter) without flashing stock OS


I want to enable /system write option in TWRP ( without flashing TWRP again ) so that :

a) Flash backup Nandroid ( of stock OS from the first Nandroid), if I decide to upgrade OS

b) If I don't want to upgrade, have a complete Nandroid and tweak further knowing that I can safely roll back changes

In other words be able to backup and restore /system along with data

Is it possible and how ( please provide detailed instructions )?

Edit: Flashing different versions didn't help - see my unanswered post on XDA
It appears that the issue was TWRP's fault. To make the /system partition writeable, one could:


Uncheck the checkbox near Mount system partition read-only, in the Mount section of TWRP (screenshot at the end of this answer);
Delete, move or rename the .twrps file to force TWRP's setup wizard to trigger, in order to make /system writeable. This file is located under sdcard/TWRP, and can be managed directly from TWRP's File Manager, accessible from the Advanced section of the recovery. Notice that any TWRP setting customization aside of themes will be reverted.