1. It's mandatory.
The player can immediately put the creature down in one of his three city districts if he has at least one unit in this district. Otherwise, the creature is placed in front of the player, on the corresponding Power tile.
2. It's mandatory.
A creature in the reserve will return to the board, in one of a player’s districts, when he uses the Recruit action. If several creatures are in the reserve, the player has to choose which one of them comes back first.
3. You merge it with the units in the city.
A creature can never be destroyed. If the last unit with a creature is destroyed, the creature his immediately put back in the city, on a district with at least one unit. If that is not possible, the creature is put back in the reserve, on its Power tile.
4. My interpretation is, that you immediately put a creature in reserve to a moved troop. First time a creature comes to the board is when you do action of buying the power tile, not recruiting. Also you when the last unit is destroyed or recalled, you immediately link remaining creature to another troop. Again not a recruit action. I would give more weight to the first part of the rules that talks about creatures.
The player can immediately put the creature down in one of his three city districts if he has at least one unit in this district.