IMHO you shouldn't waste your time with fixing this issue. I'm using lightdm with the lightdm-gtk3-greeter, it's possible to use it without a greeter and JFTR there's also lightdm-gtk2-greeter available and IIRC a few others. Btw. I don't know if there is the need to install a X driver or if it's already installed as a Xorg dependency, to get the Xorg conf for a keyboard working. I run into the same issue as you in the past, but I don't remember what display manager I used. Unfortunately my user password includes two chars that are different for my DE keyboard, when I need to login with US settings.
