Yeah, the nginx config for subfolder installs is a bit tricky 😄 Glad you got it working!
Multiple NodeBB in cluster
Is it possible to run several NodeBB instance with a single Redis/MongoDB?
Yes you just have to choose different database names (mongo) or numbers (redis)
But how to synchronize pictures and other data from ./app on filesystems?
Oh sorry are you talking about running a cluster of processes for a single forum, or running multiple forums on the same server?