A Clue All Along?

As you’ll know if you’ve read this post (and to understand the following I think you should), I believe there was something important going on in the scene in Half-Blood Prince where Harry catches Mundungus Fletcher selling Black family artefacts. The scene includes an appearance of the Hog’s Head barkeeper, who we know is Aberforth Dumbledore, Albus Dumbledore’s brother.

Well, I was rereading Order of the Phoenix this weekend and came across something that I feel could be related and/or important. In book 5, Sirius is telling Harry, Ron, and Hermione that Mundungus Fletcher had overheard them planning their Defense Against the Dark Arts group in the Hog’s Head bar:

“Who overheard us?” Harry demanded.
“Mundungus, of course,” said Sirius, and when they all looked puzzled, he laughed. “He was the witch under the veil.”
“That was Mundungus?” Harry said, stunned. “What was he doing in the Hog’s Head?”
“What do you think he was doing? said Sirius impatiently. “Keeping an eye on you, of course.”
“I’m still being followed?” asked Harry angrily.
“Yeah, you are,” said Sirius, “and just as well, isn’t it, if the first thing you’re going to do on your weekend off is organize an illegal defense group.”
But he looked neither angry nor worried; on the contrary, he was looking at Harry with distinct pride.
“Why was Dung hiding from us?” asked Ron, sounding disappointed. “We’d’ve liked to’ve seen him.”
“He was banned from the Hog’s Head twenty years ago,” said Sirius, “and that barman’s got a long memory. We lost Moody’s spare Invisibility cloak when Sturgis was arrested, so Dung’s been dressing as a witch a lot lately…”

So apparently, Aberforth banned Mundungus Fletcher from his bar twenty years ago and hasn’t forgotten about it. And yet, only the next year he’s seen having a clandestine meeting with Aberforth outside the Hog’s Head?

My question is, why did Aberforth ban Mundungus from his bar? And why did he apparently forgive that incident in the sixth book? Also, did we ever find out conclusively what tipped off Umbridge to the secret DADA plans? Is it possible that it was either Aberforth or Mundungus? Was Mundungus really in that bar to look after Harry? You’d think that his failure to do so properly once before would give Dumbledore pause in assigning him such a task again. A couple of possibilities present themselves. Perhaps Mundungus was their to spy on Aberforth instead? Or maybe he was their for his own reasons? At any rate, I think there’s more to this situation than meets the eye, and I’m inclined to think that there’s some sort of double crossing going on. I believe that Aberforth and maybe Mundungus could possibly be untrustworthy, especially Aberforth. Why is it that Albus speaks so little of his brother, especially when he lives so close by? Is it possibly because he doesn’t trust his brother either?

What does everyone else think?