If you have enforced recruitment, can you add them to a troop on the map? This applies to both buying the tile and replacing them in future.
When buying the tile, you can add the mercenaries to your city but it is not considered as a recruitment. So enforced recruitment does not work herer. Recruiting scrib does not work either.
But it works when you recruit your mercenaries (as mercenaries are considered basic units)
Can the mercenaries exist on the map alone (i.e. no normal units with them)?
Mercenaries can exist alone on the map. I guess player will use the basic units because of the color but you can have a troop with 3 mercenaries on the board.
If the mercenaries die, are they recruited like units (1 power / unit) or like creatures (free to place as long as they can be placed legally)? Can the Reinforcements DI card place the mercenaries?
I guess you're talking about recall your units after a battle. So the answer is yes, you get 1 PP for each mercenaries you recall. And yes, Reinforcement DI card works.
They act like units. In fact, mercenaries ARE units. So for example, when you use the recruit action, everything (power tile, ID...) with recruitment works with mercenaries.
Just remember that buying the tile and add mercenaries in your city (for the first time) is not a recruitment.
If you have enforced recruitment, can creatures be placed on the map?
Have fun :-)