Yes, great questions.

I think topic specificity would sort itself out naturally. People could create a censor with a stated goal/mission. Those that have an unclear or overly broad mission will not attract many subscribers/participants.

Not sure what is meant by composability, but I think you should be able to subscribe to as many censors as you want.

Concerning the curation part, I think there is actually a huge opportunity to even make this enjoyable, e.g. through gamification. Prolific 'hunters' of specific unwanted content could get a top spot on a leaderboard, and receive zaps from satisfied subscribers.