The error suggests the mongo lock file is present so mongo refuses to start up again.
If you're sure mongo isn't running, just delete the lock file.
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?