Hi Carlo,
Thank you for your answer, but unfortunately I wasn't yet able to solve the situation with your advice, maybe I'm misinterpreting something.
A small clarification first: the "/etc/profile.d/guix.sh" script installed by the Guix installation script; what I shared in my previous email is exactly as it was installed, so it wasn't myself who modified guix.sh or created those lines.
With the vanilla "/etc/profile.d/guix.sh" script (the one I shared), and after installing ipython with guix, here is the result of echoing my XDG_DATA_DIRS after a reboot, when I log in through a tty (since Gnome breaks and I can't log in there, as discussed):
Notice that "/home/tomplaa/.guix-profile/share" is repeated, and this is the reason why I previously had written that the last lines in "/etc/profile.d/guix.sh" seemed superfluous to me, because they were appending something that was already there after sourcing the guix profile./home/tomplaa/.guix-profile/share:/home/tomplaa/.guix-profile/share:/usr/local/share/:/usr/share/:/var/lib/snapd/desktop