I have had a challenge where my opponent was tough enough that we both ran out of cards well before it ended. Defensive cards like freeze will make it more likely.
I would say that, because of coaching, you want to focus on one of those girls from Indira up. My top girl gains stats 30% faster than one at level 200. Using coaching, your other girls advancement is tied to how fast your best girl advances. I'd use one that is easier to get to higher levels. You are correct about the cards, though. The only way to make them better is to stack them on one girl to get dups. Kinda makes sense to make that your best girl though (if there is a chance that defense matters in the future). If it doesn't, then I could see using them it strengthen a relatively weak girl.