Ouch at the thought of someone kicking her. I'm not even going to discuss that possibility it makes me so annoyed.
I think I'd be getting her a senior health check at the vets. (Hope you have one who is sympathetic to older cats - although 17 is becoming more common these days.)
If she started to get fur mats then maybe they worsened and became intractable too quickly for her to cope with - and I'm not that surprised she won't let you groom her now. Snarls can hurt when they're tugged - remember having your hair brushed by someone else when you were young? A fresh start might be the best thing.
Looking at her age, there are a few things your vet might want to check - she'll probably want to do a blood analysis if there hasn't been one for a bit. A dental check is also called for eg in case one or more of those remaining teeth is starting to cause a problem. (If she had a sore mouth, for example, she might not only feel miserable but might be disinclined to use it to groom.)
There's also the possibility of some arthritis as I mentioned above. I've been trying to think why she should be staying outside as most older cats do the opposite - start to spend longer inside sleeping. And it's been rotten weather if you're in the UK. I'm sort of considering whether she has arthritis and is finding it difficult to poo. If she goes outside and doesn't have a tray inside, then if she was having problems, she'd stay out to deal with them if she was very clean by nature. It's all I can think of, really.
As I said, I think I'd have her to the vet for a check up. Tell the vet everything you've told us and see what transpires - and let us know how she gets on.
Other posters may have more suggestions.
Lots of luck.
PS - maybe try some slightly sloppier food if she hasn't got many teeth left but the vet will see if she's underweight if you haven't weighed her.