I don't want to come across unsympathetic here as it really is a terrible thing to deal with. You sound like you understand the quarantine issue now so I will leave it at that. Quarantine isnt something you want to skip..sooner or latter this is what happens if you do.
PP isnt the cure all so many think it is. It doesn't kill everything and definitely not in one 5 hr treatment. Im sorry but what ever you heard about PP is wrong.
Additionally theres the question of how much you used to initially dose. Did you measure it or use the "add enough ti make the water pink" method. I also see you use "Prime powder and API Tap water conditioner"? These actually react with Pp and neutralize it. PP was really not a good treatment here. Potentially it makes it worse because it removes the fishes slime coat.. the number one line of defense it has against bacteria and virus...and it chemically burns gills which causes inflammation at a time when you don't want impairs damaged gill tissue.
My advice is add salt to the tank at a rate of 1 level tablespoon per 1 gal water. ..do daily large water changes and replace the salt you remove. Do this for the next week. Odds are this is viral and not parasite... if the fish start sliming up and shedding mucous that could be from an external bacteria thats taking advantage of the weakened fish.. if that happens an antibiotic like nitrofurazone may help... but most of the time the salt and lots of clean water and time is alls you need.
Hth
Al