1. Historically, many pirates often let their victims go - it was good "business" (in the sense that mercy usually resulted in easier captures). Since Pirate society was essentially egalitarian (the Captain was mostly "first among equals" - not commander in chief), maintaining a fleet was also problematic. Even the greatest pirate chiefs (Morgan, for instance), could only control "their" fleets if there was prospect of loot. Trying to model the reality of this might be interesting, but probably would not be very fun in a mobile game.
The real reason why this has not been included, though, is game balance. Allowing players to have fleets would tip the scales heavily in favor of piracy over trading, and there is also the question of how this would fit into the game mechanics. This is a frequently requested feature, though, so I won't say I definitely won't do it - it's just not a priority at this time.
2. I will eventually be looking at ways to make the higher levels more challenging. One of the way's that I plan to do this is to add more challenges at the higher levels (dedicated pirate hunters, famous pirates, etc). I'll tune the combat variables too, if this seems called for.
