I can help you with your questions.
1- You can only recruit one hireling but Inn or Stronghold. If you have two Inn then you can recruit two in total.
Check this out for Building and army cards rules:
With the rule above let me give you a example:
Scenario: Game for 3 players, on your city you have one Inn and Stronghold.
On your turn you have 2 gold coins so you can hire two hirelings. (One by the Inn and one by Stronghold)
Then on another player turn let's say that player didn't use a gold coin so he passed that gold coin to you to do your build/hire phase and you as lucky duck you can hire one hireling
2- That's a good question. That is a tough monster against troops and heroes, you don't need to kill your units that reach defense zero since it's a overpowering stench, you know it's awful smell from him due to had been eating bodies in the ground.
3- Yes you can convert resources even though you are not going to use it. (You need to be the active player in order to do this evil trick )
Example: You are the active player and currently you have 3 resource tokens (2 gold, 1 grunt) so you use the harbor to convert 2 gold tokens into 1 good token, in that way other players have least chances to play cards when you pass the remaining resources.
Here are extra details about Harbor card from the rulebook.