My mum recently brought a second hand car from the mechanic she has used for decades. She grew up in a setting of domestic abuse and hasn't had the best life. Due to this, she has a tendency to catastrophise and she always puts her self in the position of the victim. Consequently, when a few things went wrong with the car she purchased it was blown out of proportion.
If you gut is this woman isn't a scammer I'd go with your gut as there's lots of things that could be going on in the background for them to end up at your door.
The Money Advice Service has this advice:
"Buying from a private seller makes things slightly more complicated. As long as the product has been accurately described, you are not entitled to a refund, repair or replacement."
If your happy for the garage to look over it then do - that's the course of action I'd take. For what it costs, it gives you fantastic evidence to refuse to take it back. Plus, if she does go to the small claims court you have the evidnece that it was sold as described and you've tried your best to resolve the situation.
If everything is ok I'd then write her a letter explaining how her actions have made you feel, how much financial stress it has put you under etc. It might be useful for her and her friend to understand how their actions make others feel.
I hope tomorrow goes well for you.