I was deferred from a school after applying early decision and I don’t feel confident about getting in. My second choice school has early decision II. Would I be able to apply there under ED II after getting deferred from the other school?
Yes, you can
Yes. If you are accepted and the financial aid provided is sufficient, then you will have to withdraw your other apps, including the app at the school where you were deferred.
Yes, and as noted, if you are successful at the ED2 school, you will need to withdraw all other applications (including the one at the ED1 school.) So just be sure you are willing to give up that “dream”.