In the picture, it doesn't yet look too skinny. Good if it ate something! Have you tried earth worms? They have very good nutritional value which could support healing if s/he likes to eat them.
Is sludge destroyer to do with snails (sorry my english!)? I think you shouldn't use any that stuff with newts, they do have delicate skin.
If you put it in a small container, I'd think change clean water every day.
About the pain, I can't say how they experience it. It must be painful, but impossible to measure what's standable and what's too much. I think here somewhere was a recovery story of a newt that grew new limb (?), it took many weeks but when it started to heal, it just slowly came better and better. It's quite difficult situation for you, to decide what's best. I guess if the behaviour is quite normal and no suffering noticed in movements and such, and if digestion functions, you can wait and check the situation every day. But if it seems unbearable, then you have to be ready to euthanize.
About medication I can't help, they are not familiar and I don't even know what we would use here in Finland...