[x] Someone from another class.
Expand Shinji's close circle to other Houses. Also, Natsumi needs to gain confidence and make friends with other people too. We don't want another Hermione or Rin situation were she wouldn't be able to do much without him or grow too attached. Ernie will already be working with Shinji for the week, so why not give a chance to someone he won't see as often?
You won't always be with people you expect to be. Might be a valuable lesson to learn to work with people you are not familiar with, as well as not knowing their forte. Go in blind and hope for the best.
[x] As soon as possible.
While I would like to double on the yolo, we have to take in account that Shinji will be fairly busy this week, getting familiarized with his environment, on top of all his first pile of homework, including working on the Potions assignment. Time spent preparing for DADA will mean less focus on this. And since it's a four-people cooperation thing, I would rather succeed this one at the risk of failing the other (two-people assignment).
Best tackling the DADA examination asap, so he can secure a more peaceful mind during work on his cure for boils. If he stresses over what's going to happen during DADA, I doubt he will be able to provide a satisfying result. Shinji won't be as prepared for DADA, but that's usually what happens when you deal with dangerous scenarios anyway. You need to think on your feet. Better to learn that lesson early than late. So half-yolo, I guess.