Originally Posted by Nidhal AKA the king
BTW: Bootloader unlock allowed: No (is it possible to make it Yes somehow?)
No, no way.
To debrand your phone use latest flashtool and flash a FTF from Mega Collection for your Model C6903, there is no any issues as they are Stock and made for your phone. The R1B or something else is just a customization, not even noticeable.
Flash Generic/International and you are Fine.