edit: checked the game again, I used shadowplay already in the move before (moved twice). Imho "(once per turn)" relates to to "exert THAT minion" not general to all minions. So exert the minion that has been played but only once, meaning you can't exert a hobbit again to wound the same minion.
If it relates to all minions it should not be activated by gemp more than once every turn...
No, each minion is played only once, so there would be no way to exert it twice, even if it had no "once per turn" clause on it. Therefore the "once per turn" refers to the exertion of any minion.
You are incorrect, you can use the
Shadowplay for each minion played (pay the cost), but you should be able to exert a minion only once per turn. I can't make it not activate on any minion played after it's used, because there might be an advantage to exerting a hobbit and FP player should be able to do that, without getting the
Shadowplay effect.