I'm hesitant to suggest, and please know there are so many more experts here, trust the moderators and experts first! That said this is what I usually use melafix for, situations where a clear diagnosis isn't possible, there may or may not be an infection but a 'boost' feels like a good idea. The commonly sold Melafix is really just tea tree oil (melaluca alterniforia) in a suspension to disperse it through water. It is a natural anti bacterial and antifungal, I posted a thread asking for opinions here based on experience and the feedback I got mirrored my own experiences, I wouldn't suggest dosing the whole tank but it could help the betta in his quarenteen tank (just watch levels & dosage and maybe add an airstone to the water if you have an extra) melafix actually does better with out carbob and can impact you beneficial bacteria so if the Betta is still out of the tank, it might be the best place to treat. I don't know of any impediments it would place on true RX therapies...doesn't mean there aren't any, I just haven't experienced them....I use tea tree on myself and my kids with the md & pharmacists kept in the loop - no issues with other antibacterial treatments in mammals but we use it externally, so a more educated/experienced Aquarius may have better info...
Hope things are going well