Yes, that should not be a problem. My rather elementary understanding of the GPL suggests that it allows you to use it for whatever you want to use it for, even for a site that is behind a paywall... but the source code has to remain open-sourced.
If you have no intention of forking and selling a customised version of NodeBB, then I think you're pretty much in the clear
Not really -- as mentioned prior, the reason this is impossible is because the redis database doesn't usually live in a place that is readable by NodeBB.
There are ways to dump Redis data into JSON, so all is not lost.............