SamyGO

You can't change a file from it. If you want to change there is 2 way.
Using mount --bind, you can point a different file for that file. I think this makes the job well. But needed to mounted at every boot up. You can do it by Advanced script.

Other approach is updating the /dev/bml0/6 partition. For doing that,
*you needed to dump your root image
*move that image to your PC
*modify it via SquashFS/FAT image
*and flash your flash chip.
*pray to god.( Always needed specially while doing critical things)

I haven't try unmount root partition before. I don't know what happens. First try it.
AFAIK, TV's firmware update process doesn't unmount that device before flashing.
You needed to stop watchdog before. And you have to know that If you make a mistake in new/modified firmware, than your TV could be bricked. It's probably could recoverable. But It's better to have all partitions image as a backup.

If I were you, I will
*Revert firmware to previous one
*download last firmware of TV from Samsung,
*put modified root image dump as "rootfs.img" to the /image directory, near exe.img.enc on new FW that gonna flash.
*update validinfo.txt file and add requied changes that described at bottom
*than flash it as usual firmware update.
*pray to god.( Again, not forget )

This will safely flash "rootfs.img"
You could needed to update validinfo.txt file too.
Its structure is basic, for example: *007_exe.img_35bda3e4 -> star, partition number, underline, image name, underline, crc32 at lower case.
Your new valid.txt needed to be like: If your TV is not bricked after flash, do not forget that you are praying a minute ago and say "thank you God!". This will prevent your TV to become bricked next time.

Read only? Than change its permission!
I don't recommend you to modify your firmware if you don't know such basic steps of linux.
Instead please use "mount --bind" approach with "Advanced Mode" of SamyGO firmware patcher. This will automatically mount your file at every start.

Please read wiki page about modifying images "Mounting the Images" section. Depending your image, you needed to mount as FAT image or extract it as SquashFS image...
Playing with Firmware Images
If you can't than you needed to search on web for that basic steps.

I think I understand your problem. RootFS is in SquasFS, and SquasFS images are always read only if you mount. You needed to extract it, change required file and recompile the image change...

You needed to preserve same format for SquashFS, like using no LZMA compression, little endiennes... etc. Search on forum and wiki also on web to identify SquashFS format from original image and how to create one again. Please drop lines here later too for other people.

Size of flash don't indicate something. You needed to sure using right commands while creating squasfs image. I don't remember what they are, you can find easily by searching. Exact file size is not quite possible if you change something. Because SquashFS use compression, definitely it will change on size.