[Q] Reboot when altering system files topic

I have two separate issues which I believe are caused by the same thing.

1. When I try to alter platform.xml in the /system/etc/permissions folder via ES File Explorer to enable 3rd party apps to have write capability to the SD card, the edit is unsuccessful and my phone reboots. When I pull a copy of the file to my PC, alter it, and then try to push it via ADB, the same thing happens. I previously used the ADB pull/push method to edit my hosts file and had success with that after doing it while in Recovery, but this time around both ways (in recovery and with the device booted up) result in my device rebooting. For the SD card fix, I've also tried the app that does it for you and encounter the same issue.

2. When attempting to update the SuperSU binary, the update fails my device reboots. This happens with either update method - normal and recovery.

So both issues seem to have the same root cause... my phone will not allow any editing of system files. Anyone have any suggestions?

For reference I'm on a Galaxy Note 4 (model SM-N910T) on T-Mobile.
Baseband version N910TUVU1ANJ7
Android version 4.4.4 (stock ROM)
Kernel version 3.10.0-2796035
Build number KTU84P.N910TUVU1ANJ7

I'm also running Wanam Xposed - not sure if that has anything to do with anything but just fyi.

Thanks