This post explains how to build GNOME Shell from source thus preventing Unity wreckage. There's also a line which sets Shell as the default window manager at startup. I was wondering, would it be possible to create a new session which loads GNOME Shell on startup while keeping the other sessions (Unity and Classic GNOME) unaltered and usable? Thanks for your help!
This answer and another answer to different questions give instructions on how to define a user defined session that may be added to the list of sessions at startup from gdm. We need to add the setup as it was detailed in answers to this question.