Under both types of session there would be redundancy, since sometimes Default means Adwaita and sometimes it means Yaru, but I see that as a lesser problem than being unable to select "the other" theme.
Finally, a minimal proposal is to create just one of the symlinks to get Yaru into the list but not Adwaita, so only yaru-theme has to be patched.
Just to make it crystal clear. I now enter my Ubuntu session, go to Tweaks and get this shell themes list:
(1) dark-compact light-compact
Default
Materia
Materia-compact
Materia-dark
Materia-
Materia-light
Materia-
Nevertheless, Adwaita shell theme IS installed in my computer. Hence I would like to see:
(2) dark-compact light-compact
Default
Adwaita <-------
Materia
Materia-compact
Materia-dark
Materia-
Materia-light
Materia-
If, instead, I enter my vanilla Gnome session I again get (1) above but I would like to see:
(3) dark-compact light-compact
Default
Materia
Materia-compact
Materia-dark
Materia-
Materia-light
Materia-
Yaru <---------
Now, creating the symlinks I proposed, I won't get (2) and (3) but the quite similar (4):
(4) dark-compact light-compact
Default
Adwaita <------------
Materia
Materia-compact
Materia-dark
Materia-
Materia-light
Materia-
Yaru <-----------
Under both types of session there would be redundancy, since sometimes Default means Adwaita and sometimes it means Yaru, but I see that as a lesser problem than being unable to select "the other" theme.
Finally, a minimal proposal is to create just one of the symlinks to get Yaru into the list but not Adwaita, so only yaru-theme has to be patched.