/system cannot be remounted as RW, even when rooted (Sony Xperia E3)

Using Total Commander to remount the / directory as RW also remounted /system as RW.

Howto:

1. Enable Root Functions Everywhere in TC settings.


2. Proceed to add a new button to the toolbar with Function Type Internal Command.


3. Press on the >> button, and select the 119 Remount option.


4. Press OK/Apply


5. Navigate to root (/) directory.


6. Press newly created Remount button and accept any prompts.

Edit: I've found that when I'm using Kingroot to manage root, mounting works correctly. When I migrated over to SuperSU however, I could not remount / or /system as RW.
If the above steps don't work for you, Kingroot might be worth a shot.

Another edit: While I do recommend Kingroot as an option to allow access to your /system folder, you should exercise caution with Kingroot, as it is known to gather information about your device. I would save Kingroot to be a last resort, should you cannot find any other method of remounting /system as rw successfully.