When you play data theft, do you have to choose a card you could legally play? For example, could Leia play data theft and use grenadier even though it's a trooper card? The wording "play that card as as if it were in your hand" seems to me to suggest a figure would need to have the trait associated with the card since if it were in your hand only certain units could play it.

I also interpret it the same way. You don't have to choose a card you can legally play, because you just choose a card. You play it later in the round, but then it must be legal.

yup, Leia could choose Grenadier for instance, and then a rebel trooper could play it later in the round