I wonder if this patch (done properly) could make it into the official repo? While it is a workaround for an LG TV bug, I am not sure the possibility of engaging PC mode would ever be preferable for a Kodi distro.
Reporting a PC style device as a PC style device sounds like the correct behaviour.
The TV not allowing the user to choose picture enhanced based based on this seems like the bug.
I'd be nervous about changing the reported source type for everyone. I believe using PC mode will often default to a non-overscanned display which is desirable and the change you are suggesting may make things worse for others.
The correct solution is to complain to LG and hopefully a firmware update will allow your use case to work.
The workaround should be that the kernel defaults to PC mode. but allows a runtime override (perhaps through sysfs) to change the SPD_SDI type. A display settings option in kodi could allow setting this. But that is more work, and really needs to be done in the upstream linux and kodi repos, not by LibreELEC.