I wouldn't even try to demat this cat. The best solution is to take her to a vet or cat groomer, have her sedated and shaved. She's bad tempered because she's in pain from the mats that are making her feel miserable. Once her coat starts coming in, keep it mat free by running a steel comb through it
once a day gently, and follow by treats. She will gradually come to enjoy grooming rather than hate it. This is the type of comb that works well with longhaired cats:
http://www.pet-dog-cat-supply-store....-flypage-31097
As far as a cat on a vegan diet, that's absolutely harmful and non-sensical when a cat is a carnivore and needs meat to thrive. Dogs are omnivores and can veggies, but cats shouldn't be forced to eat this kind of diet. She will lose weight if she's on a proper diet and not overfed, but portion size should be reduced gradually over many months.