To find all instances of where a function or variable is used in UE4, right click it and select Find References. Then click the binoculars to search for it in all blueprints. You will find that it is only called in the event graph of BP_Unit, in the Kill Unit comment box. In your case I would replace or modify the function so that instead of setting GameOver to true etc., you set a boolean in the game mode, which you check when the turn ends, and then spawn the new units if it is true.
That is odd. Retargeting worked without issue when I did so. Maybe the default mannequin has been changed since then. I’m looking at the skeleton, but cannot find the root you are referencing. Could you give me the repro steps?