Welcome to the forums, sidney24, and your sweet little conure, although I am very sorry for the worrying circumstances. Don't forget to tell us her name btw, will you?
Was the vet that you took her to a certified avian specialist? I only ask because it just feels like the fecal test alone does not seem to be sufficient testing to try to pinpoint what may be causing this destructive behaviour - unless they ran blood tests etc that were not mentioned. Many avian vets will offer you more in terms of ongoing management of a situation like this too. Just in case they were not, or you might like to seek a second opinion, the link below may help you to find an alternative...
Plucking can be caused by anything from boredom, stress, sexual frustration, dietary deficiency, parasites, neurological issues right through to having been weaned incorrectly as a baby or even taken from it's parents too early. These last two possibilities can have psychological effects that only manifest themselves later in behaviours like plucking, biting and screaming. One of our esteemed former moderators began a thread which describes some of the issues and resources to attempt to help mitigate or end plucking:
If you've spent very much time with parrots you've probably heard the term or seen the results of plucking. Plucking is disturbing and complex, in many cases the reasons are difficult to pinpoint. There are no easy answers or explanations. It's a terrifying event to discover a pile of feathers...
www.parrotforums.com
I have never had a plucker so I can only imagine how devastating it is to witness a beloved bird driven to do this to herself for whatever reason. I did, however, once have a Quaker whom i suspected was beginning to show plucking behaviours and immediately took him to my vet. It turned out that he was just storing pellets in the feathers on his back (bizarre!), but my vet clinic had a whole armory of treatment and management strategies that they would have brought to bear if my Quaker
had turned out to have been plucking. Unfortunately I don't know what they were because it turned out he wasn't plucking, but it just leads me to think that perhaps your vet should have had a few more ideas to support you with this. Perhaps it is time to seek a second opinion.
I hope some other members will be able to give you some adviced based on their own experiences. Thank you for joining, sidney, I'm so glad you found us, and I hope that you can find some relief for your little one very soon.