Have you tried chewies? Silicone chewable shapes for sen, amazon do them. Dd2 chews her fingers loads and currently has a horrid rash around her mouth from chewing her fingers and dribbling. She needs encouragement to use the chewies.
I think they are quite hard wearing, but the texture might be harder. Dd2 loves chewing tags on things, she used to just touch but now chews. You could just keep encouraging with a chewie, they are on a "string" which dd2 loves to chew too.
Best of luck with the diagnosis, and so sorry about your parents being unsupportive. Dd2 has ds and possible asd. I know what you mean about the telling off, it just doesn't work! If I get really cross she doesn't understand and just gets frightened that I've got very cross. I end up just being constantly repetitive and trying to be endlessly patient.
I bought two chewigem necklaces for DS1 when he was age 6. He tried one and complained it was too "plasticky" and went back to chewing his clothes.
He's now 10 and still chews holes in his clothes. I recently found the unused chewigem still in its wrapping so gave it to him thinking it would be in the bin before the day was out. He loves it! Wish he had at age 6 it would have saved me a fortune in clothes.