In regards to pint 3, the killer apps will probably be closed source. At the moment there is a lot of cooperation to build the protocol. In the future they will be like competing email providers.

That apps that likely make nostr grow, won't be social media, but something like a music streaming service that allows you to view trending songs of the people you follow or a restaurant review app that lets you see the social graph of the reviewers to determine if they are legit. The social media apps will be the home page of the nostr ecosystem to enable discovery

The network effect of all these apps is cumulative