The AU accounts will help lower your utilization. You are probably taking a fairly significant score ding from your single account (closed one, maxed out) being at peak utilization. It would definitely be in your best interest to pay that account off ASAP, as it will do wonders to your utilization.
True. We would need to know about the presence of other negative items to determine if eliminating that one piece would result in a significant score gain.
If you have major delinquencies (anything 90+ days) like 150 day lates as recent as a few months back, the elimination of any other major negatives like a collection that's older would likely have zero impact at all on your score unfortunately.
Might I also suggest hitting up the rebuilding forum and looking into ways you could possibly clean up some of your dirty accounts, thus reducing the amount of time they'll adversely impact you.