you can install trisquel-mini?
I have found that if you install a new desktop on top of your previous one it
will work if you choose the other desktop on login. Where I find that it
starts failing is when you have more than 3 different desktop enviorments.
I now have trisquel-mini and I installed all the default gnome settings
(including gnome-shell). This has worked for me in the past and I could
choose to login as gnome-shell or lxde. But sometimes minor configurations
need to be changed or .configure files need to deleted.