Q: If I draw a Creeper while executing a 5 Goal Mission, can I choose to play it last, so I can win with some other card first?

A: No. Unfortunately, you can’t choose to play the Creepers last. As Creepers they are never really held in your hand (or temporary hand). You have to play them immediately, as usual, the only difference with 5 Card Mission is that, unlike in other similar situations, you don’t get to draw to replace them with non-Creepers.