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.

Note that 5 Card Mission is the Meta Rule equivalent of the regular New Rule Goal Bonanza, and the Black Knight expansion for Monty Python also has a New Rule called It’s a Quest, which works in much the same way as Goal Bonanza (except it’s discard 3 Goals, and draw 4 cards to play).