It sounds like you waited too long to resolve this problem. I would recommend seeing a vet, for two reasons
– they might be able to sedate her so she can be cleaned up and groomed properly
– they might help you determine whether her distaste for grooming stems from pain which can often be the case
If pain is at the root of the problem, it needs to be addressed. If there is nothing medically wrong with her, you need to start training her to accept care right away. Check out Donna Hill’s youtube channel, she’s got some great videos showing how a dog can be condition to accept all sorts of care including teeth brushing, nail clipping … the principle is the same for all such things.
Jasmine had IBD caused by food allergies, which is likely the case with your dog as well. There are reasons behind prescription foods, they are easy to digest, provide all needed nutrition and some of them are either hypoallergenic or limited novel ingredient. With IBD, if offending ingredient gets reintroduced, you might end up with a flare-up again.
With Jasmine, we managed her IBD with home-cooked diet, herbal therapy, probiotics and digestive enzymes.
If your prescription food is hydrolyzed, you’d have to carefully try novel ingredient diet (ingredients your dog NEVER had before). Accomplishing this with home-cooked diet is easier than commercial foods, because with hopefully exception of the prescription foods, it has been found they contain all kinds of ingredients not listed on the labels (likely from insufficient or no cleaning of the equipment between different recipe batches). So choosing such food you’re risking your dog eating the very ingredients that are behind the problem. Home-cooking, you have full control over what goes in. For that, you could work with BalanceIT to get recipes as well as balancing supplement. You could work with co-ops to obtain the ingredients you need for reasonable price. And you could cook relatively large batches at the time and freezing it.
I’m afraid that anything else you try might lead to flare-ups unless there are some manufacturers out there who guarantee full ingredient control over their formulas. Perhaps somebody like Honest Kitchen or Lucky Dog Cuisine might but I don’t know whether that would come cheaper or not.