It's not looking good, though it could bounce back. I would switch to Septra. Typically if the nem doesn't improve after a few days with one antibiotic, I will switch to the other. Your nem looks like it's been in a somewhat steady decline rather than showing a marked improvement.
Make sure that you do 100% water changes. I recently found it to be essential to removing any pollutants from the QT. It seems that the nems like fresh salt water and seem to perk up after water changes.
I would keep the nem in QT. It still needs treatment, and just because the DT may have stable water quality, the antibiotics are what's going to make a difference at this point. Plus, if you have clowns in your DT, they'll probably bother it rather than helping it recover. And, if the nem continues to decline, it's easier to remove the nem from QT than risk it fowling up your DT.