"Before you play a Yellow card, you may exhaust this support to give it the Ambush keyword."

Now I read this to apply all within one action, but it could also be read as one action = play infamous designating your next yellow card to have ambush. Then on your next action, play that yellow card and resolve the ambush.

My feelings on it are if the play were to be divided into two actions, the card would say, "give the next yellow card you play Ambush."

Thoughts?

Giving up an action to add Ambush to a subsequent action is hardly worth it. Not to mention, it becomes a bit of a bookkeeping problem to remember which card has the temporay Ambush trait. I think your first intuition is correct.

"Before" happens immediately prior to the condition.

Infamous is a support, so you play it on one turn then can activate it later based on the trigger condition. Then you can activate it repeatedly as long as the conditions are met.

So one action to play Infamous, then later you use an action to play another Yellow card. The "before" trigger then happens and you can exhaust Infamous to give the card you are playing Ambush. Then your next upkeep Infamous will ready and you can use it's ability the following turn.

Before abilities are triggered not action abilities so they just happen and don't cost an action.

In the case of Infamous, before the yellow card you are playing happens, Infamous triggers giving that card Ambush this turn.

I could also see it being really powerful in a Han Solo deck.