My read is that the extra damage from using Feint only applies to the next Wound-dealing attack the target suffers.  Now, said Wounds could be from the Duelist themselves, or from an ally, as the Maneuver just says "if your target is injured again" without specifying who has to be the one doing the injuring.

Pretty much Raises, once you roll your dice, fall into the "use them or lose them" category.  There's really no "holding your action" in this system.  That being said, the PC can spend their Raises to create Opportunities for their allies (or even for themselves) or to apply Pressure to the Villain, forcing them to act a certain way or else be forced to burn an additional Raise to act differently.  Neither option direclty inflicts damage, but sometimes the best way to win a fight isn't to beat a Villain into a bloody pulp, but to get them to give up on their current goal and leave the scene, something that Pressure, Opportunities, and clever thinking on the part of the players can accomplish.  After all, it's one thing to say, "yes, we beat the Duke's top assassin into the dirt" but quite another to say "yes, we thwarted the Duke's top assassin in his attempt to slay Madame du Paix by wrapping the assassin up in a curtain before deftly manuevering him onto an impromptu catapult that sent him careening through the night's sky and out into the bay."

But generally speaking, Heroes in this system are meant to be active, and should generally be doing something at any given point in an Action Sequence.  To be honest, cases of a PC saying "I want to wait until Villain does this thing!" are probably best handled via Pressure, such as "I make a wild flurry of attacks, putting the Villain off-balance so that on his next Raise, he can only defend himself from my wild assualt."  In this case, the Villain uses their Raise to "defend" themselves (note this does not mean having to use Parry or Riposte of said Villain is a Duelist), and if they want to do something different (such as make their own attack or just simply flee the scene) they would need to spend two Raises.

It can be a bit of a trick for some players to shift their mental paradigm from the sort of "tactical simulaitionist" thinking that most RPGs engender and instead embrace the "this is my time to do something cool!"  Thankfully, the initiative system in this game is very flexible as to when which Hero gets to go when if they've got the same number of Raises, as they can just decide amongst themselves who gets to act in what order, allowing for some very clever planning on the Heroes' part.  While a lone Duelist Hero is a force to be reckoned with, she can be even more of a threat to the Villain if she's got allies at hand who are willing to make liberal use of Pressure and create Opportunities (a Hero with the Team Player Advantage can be fucking amazing at that last part).