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

I rooted my Sony Xperia E3 (D2203) a while ago using Kingroot, which worked flawlessly. Before rooting my device, /system could be accessed, which is how root was installed in the first place.

However after rebooting the phone, I now cannot mount /system as rw, as the kernel mounts it as Read-Only and gives me a Operation not permitted error whenever I try to remount it as rw. Apparently the kernel on Sony devices detects root and mounts /system as Read-Only.

I cannot flash a new recovery as the bootloader is permanently locked.

What can I do to mount /system at startup as rw, or mount /system when the device is online?

A non-destructive method would be great.

Here is the command I'm using to try and mount /system:

mount -o remount,rw /system

Yes, the terminal is in root mode.

Answer

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.