It sounds like your father, at least, has interpreted the talk you and your sister had with him and your mother as, "You're making stuff up. We don't want to hear made-up stuff." That's why he got on the defensive immediately when you first brought it up ("But she is that bad! Let me give you a billion examples to prove it...") rather than responding to what you actually said. And coming over today with printed "proof" seems to be him trying harder to show you that he's not making it up. Your mother's probably of the same mindset, but she didn't go the extra mile to show it, so it's not as easy to say.
Unfortunately for him, he's missed the point completely. I'm sure you were very clear, OP, but he's stuck on the idea that there is something wrong with your mother and he has to prove it for whatever silly reason. When he's told that he's behaving badly, he responds by deciding that he needs to show that your mother is worse. He's really fixated on finding reasons why she's bonkers. It doesn't matter what the problem is--he needs to stop making it the problem of you, your sister, and his grandchildren. As you've said, if he wants to talk, he can get a counsellor, not rely on his daughter.
I think refusing to discuss it with him was a wise move. You don't want to reenforce this behaviour at all. I hope things start to go more smoothly as you "re-train" them until they realise how to behave in a more civil and courteous way for their family.