TWRP Can't mount system ZR LP 5.1.1 Rooted LB

I just got locked out of my phone while trying out lockscreen apps, and then when I tried to restore using TWRP, I got the message - unable to mount system as rw. Earlier, while taking a backup, mounting as read only was solving this problem, but obviouslyread only would not work in restore operation. I used PRF Creator to create a prerooted firmware. I used NUT's latest dual recovery, BETA-SuperSu 2.49 zip and the Indian LP 5.1.1 FTF that I downloaded using XperiFirm. I threw in the Xposed Framework as an added zip as well. Is this a common problem with TWRP or was I facing something specific? Is there a way to mount system as rw from within TWRP using the command shell?