Once the quest is at the "complete" stage, doesn't let you abandon.

Best you can do is post in bug reports and fill out the template. Make it detailed, but don't be long winded. Boil it down to something like "Accept <quest name>" "Complete <objectives>" "Attempt to turn in <key item> to <NPC>" "Disconnect before item is submitted" "Log in, find <key item> removed from inventory" "Journal says <quest> can be completed, <key item> cannot be reobtained, unable to progress."