All wild caught rays should be treated for internal parasites before sale, few sellers actually do this, some sadly don't have the necessary experience to realize its a problem.
This ray needs a course of Prazi, if its feeding on all the foods you say that is a massive step in the right direction, especially with newly imported Flower rays which are notoriously fussy feeders.
This ray was obviously in a poor state when you bought it, they do not deteriorate so much in such a short space of time, the move from your dealer has simply proved too much, you have done nothing wrong.
Keep the water pristine by doing plenty of partial water changes, get the ray to eat as much as possible, 3 small to medium feeds per day will be better than over loading its stomach, too much too soon could have a detrimental effect, tweak the temp up to 28 deg C, P. schroederi do well at the slightly higher temp. I have seen these symptoms several times (this is the worst case) in all cases the ray was in a weakened state and the tail twisted as with your Stingray, be patient, recovery is slow it can take a couple of months for the tail to right its self, it won't happen over night.
Good luck and keep us all posted with any changes.
, thanks
