I know all about stupid neighbors!!
The only way my neighbors and I speak to each other is by screaming over my fence or when the police have been called. I am SICK OF STUPID NEIGHBORS!!!
I feel for you!
The putting the poop on your doormat would have been the last straw for me. I would track her down and talk to her face to face. Ask her for her phone number and everytime you go outside with your dogs call her and let her know she can follow you or watch you to prove to her that you pick up your dogs poo. I know you shouldn't have to do that, but when you're dealing with people that are that stupid you have to hold their hand through it all so that they understand!!
Best of Luck to you!!