Keep a look out for when the fish go to the toilet.
Good luck sussexgirl.
If she happy in the container leave her there bless her.
The worming med does it say in the instructions if your ph has to be above 7 to be affective.
Have you noticed the platy going to the toilet, and is her anus still enlarged.
Well if you leave her in the container she would need an airstone with meds in the water.
I think I would add her back to the tank with the worming med.
With her being thin, an enlarged anus, and white poo though usually stringy still pointing towards internal parasites sadly.
Good luck.