So much of what you say about your mother rings true for DH's mother. Your mom seems less paranoid, though, so at least she has that. (Not much.) I'm sorry that you got snookered into lunch with her. I'm sure that PC meant well.
I went through the same with my mom about 6 - 7 years ago - making contact because L had guilted me into it. It was beneficial in that I can look back and know with absolute certainty that cutting contact was the right thing to do. Of course, it made me feel bad at the time, and I wish I had not subjected myself to such a difficult lesson - I did not need a remedial course. But a lesson learned is never wasted. I hope that Matt and PC are learning as well.
DH's mom is "hosting" M and his girlfriend at this point. She is also without teeth. It took M months to figure out why she wouldn't eat good food. We told him to make an appointment for her at the Dental School and make sure that she gets there. (This is what DH did for her in the day. He paid her bills in PDX from NJ when he was in college, among other things.) She's to the point that she needs to have them all pulled. She can't chew anything. But in your case, none of you are sleeping in your mother's house, so none of you owe that to her.
That's the difference between what my M is doing and what you and your family is doing. M owes his grandma for the roof over his head - I'm humiliated that he's there, from afar. None of you owe your mother a thing. Where she is at is a sad thing, and there but for the grace of God go I. I hope that when I am old and should I ever be alone that I will not isolate myself as your mother has. Her isolation is hers. She must own it.