I chose #3.
Scope - This is a medium-size problem, as if the friend who is making substantial progress is threatening to leave and everyone else isn't doing anything, basically nobody is going to be in the collab.
Solution - Make progress checks. This is what I did with my collab. just @ everyone and say "hey, i'm introducing a progress check every x weeks. if you don't have at least y seconds of gameplay or decoration by then, you will be kicked out of the collab."
if they still don't comply, post the collab on a gd server where you can advertise collabs (eg this server or gd creator server) and get people that are actually interested and will make stuff.
#Solving a Problem (Optional)
hard forum
scarlet relic
-ex pass #1433563476462407893 solving a problem
hallow crypt