Managed to successfuly unlock the phone - SFR GT-N7105 LTE.
It worked to downgrade straight from 4.4.2 to 4.1.1 from EE, which I got from sammobile database, EVR. Weirdly, 4.3 and 4.1.2 failed - thought about trying to do this step by step, until I tried 4.1.1.
Used TWRP and SuperSU to root, everything done with Skipsoft Android Toolkit. After downgrade you have to root again and probably you will want to flash it into 4.4.2 - I recommend fresh ROM, thing which caused simlock might come back if backup was to be used, probably.
My research into what caused it makes me believe it was Facebook Messenger app, that is the only recent thing to ask me for permission to have access to SIM card - this if done uncorrectly, causes phone to lock up, with unlock codes (probably) NOT WORKING.
And few stupid problems I had which gave me a bit of a pain:
#1
If custom recovery fails to install, then do as instructed in Skipsoft toolkit with option which mentions to use it if it fails from main menu(probably 7).
#2
Phone not properly detected on PC - go to Device Management and set specific Android Samsung Driver(not exact name, but something like that) to use it by manually choosing it.
#3
Always make sure that your phone has USB debugging ON and your PC authorized(this not needed at 4.1.1 though).
#4
If phone is stuck at bootloop (samsung logo constantly flashing for few minutes +) after downgrade, then get into recovery mode (you will notice that it got back to factory) and wipe data and cache, then reboot.