I've been wondering what would happen if I moved in all garden club members, let them age and die.
The Garden Club is not covered by norespawns because the Garden Club does not "respawn". They function more as NPCs than as townies.
1) Are they still adressable as GC members when playable (buy potion, apply for membership etc.)
No.
2) What happens when all of them are dead?
Nothing.
- Will new GC members spawn in spite of notownieregen, because they're not real townies?
Garden Club members do not spawn unless you create a need for them to exist.
- Or will the GC cease to exist? And would this FUBAR my game?
The Garden Club will cease to exist until such time as you create a need for them to exist. If no one ever calls for them, they will not spawn on their own. Like with NPCs you have deleted all the examples of, they will simply remain nonexistent until such time as you create a need for their existence.