You appear to have unfortunately been taken in by a breeder, who sold you a small sick kitten for a huge price and whose only "solution" to your problem is for her to take the kitten back, knowing full well that you will have become emotionally attached and want to keep it, so it is win/win for her.
You are not alone this seems to be the modus operandi for a section of the breeding community and if you question it, they would not see what they are doing wrong...
"You had your chance, it is not the breeder's fault you will not return the kitten..."

I hope with the help of your vet, you can turn the corner for this little one. He has youth on his side, with veterinary expertise and your loving care and knowledge he has every chance.

I think you are right not to change his veterinary advised diet.
Raw feeding, needs a healthy cat. It is possible for cats to get bugs including Salmonella from raw feeding and getting the nutrients quite right can be an ongoing challenge.
He is a sick little cat, he needs all the balanced nutrients he can and that is served best by a formulated commercial diet here.