Do I need to enter a subject line when im in the email editor if im doing a subject line a/b test?

I don't believe you "HAVE" to -- but it does save a step later. Basically it pulls in whatever you have as your "A" for the A/B test -- think of it like your best bet, or existing champion... then you test against it.

Yes, because you can't approve an email asset without a subject line. You can certainly override whatever the subject line is in the testing part of the email program though.