[karmakaze]

Was this structure different in the past? Or does it not apply to the selected desktop/shell for popular distributions?

I'm specifically curious how with such a structure a thing like Gnome Unity could have happened? Was this the case with 'a developer' (Canonical) didn't follow the guidance of the Gnome Foundation/Design team, or something else?

Similarly, how about the Gnome 2 -> 3 transition? That's not one that individual developers can mix-n-match. How is the future mainline decided in these sorts of cases?

[TingPing]

They absolutely mixed and matched during that transition. People involved just usually agree on direction, and when they don't new projects spring up.