[SOLUTION] Another soft brick fix
This is for any other weary guy who ran into a problem similar to mine.
Flashed the MJ7 BTU firmware to my Hong Kong N9005 today.
It flashed the kernel and recovery but the moment it reached the system, it gave an error saying 'invalid ext4 image'.
Tried a couple of more BTU firmwares to no avail.
Installed a custom recovery (tripping knox) and then a prerooted MJ7 firmware through CWM. It ran into a bootloop.
Then a custom rom based on MK2. The rom had a lot of lag and calling didn't work at all.
Flashed another custom ROM to the same effect.
After flashing a lot more stock firmwares (none of which ever got across the 'invalid ext4 image' error), I finally tried flashing a pre-rooted ROM (MJ7 via Odin) from this
Even it ran into the same error on trying to flash the system but before trying to flash system, it flashed everything else. This updated my baseband to MJ7 (was MI5 or MI7 before this. Don't remember).
After this, the custom roms work fine, though I still cannot find a way to flash stock roms to my note 3.
Press thanks if I helped.