Have you ever tried Laxatone for her hairball treatment? It's available at most vets offices and may be at PetSmart as well(?).
Some cats will eat it directly from the tube .... if not, you can smear it on her nose or front paws and she will clean it off!

Those hairballs can cause vomiting and constipation both!
Poor baby, Luna! I hope she gets feeling better really soon!!!!