1. Transfer interrupted means the upload is now inactive (for a couple of possible reasons) kill it if you like, - you don't have to.

2. Complete means the upload is inactive, too.

3. 0k uploaders are a GUI bug, they are displayed but they are really inactive.

4. kill it if you like, it's usually a greedy client trying to occupy more of your upstream

5. The most efficient setting (or what the developers consider the most efficient setting) is always the default setting. That's why they made it default.