Downloads remove on quit choice
 

Right now, all we have is "one day", would be nice to have choices even shorter like "upon quit" and/or "upon successful download"

I agree both would be handy. You can customize the time through a defaults write:

defaults write com.omnigroup.OmniWeb OWPersistentDownloadsTimeout -int 604800

the number at the end is in seconds so doing 1 or 0 would make it disappear right after it finishes downloading.