I would try to find a private salon that would be willing to work with you I would explain the situation and definitely sell some of your stuff if you have too. A shave down is the only way and I would stop brushing out the mats ASAP because you’re causing her more pain by pulling on the mats, which are already painful as is and already tugging and pulling her skin. I hope you can find a groomer who will be willing to do it for cheap and wave the dematting fees once you explain the situation