I think if I remember correctly some posters give their cats with runny poop butternut squash baby food or sweat potato. If I'm wrong hopefully someone will correct me, but I think that's what they've used.
As for your matted girl, maybe some Rescue Remedy in her water will calm her down enough to enable you to handle her. I've got a semi-feral of my own and I've been able to get hold of her coming from behind. I grab her with both hands under her arms and hold her firmly but not too tight. She growls like the dickens but when I get her on the counter for her medication I can administer her medication pretty safely. I give mine Prozac which my doctor recommended to help keep her anxiety, stress and aggression down while I attempt to integrate her to the indoor life, as it is no longer safe for her to live in my complex. I don't know if this is an option for you.
I hope you can get hold of her to shave her. It must be painful for her to be so matted.
There are only two rules at my house: House rule #1. Cats rule. House rule #2. See rule #1.