You know I always thought that Eustace always really played down his realtionship to the Pevensies (I mean if I was related to really famous Kings and Queens I'd be making more of a big deal about it) This always struck me as Eustace being very modest. Back to the questions though - I think Edmund would have told Eustace everything by now but I'm sure Eustace felt Edmund didn't need defending as he more than made up for his poor behaviour at the beginning when he reformed and became King Edmund the Just

Well, I don't think Eustace would've known, as he even said, "Don't tell me about it," after Edmund called himself a traitor. Was that in the FotF Radio Drama or the book? Or both?
And also, Edmund probably didn't tell Eustace everything, because Aslan said "no need to speak about what has past."
That's my opinion. I always thought Edmund only shared with Eustace about Aslan and faith, not about what he did.

"Don't tell me about it," after Edmund called himself a traitor. Was that in the FotF Radio Drama or the book? Or both?

That is in the book and I can't remember if it is in the FotF dramatization of the book.

On to the question at hand, No I don't think that Eustace was thinking about that. He was more interested in conspiracy against Caspian for I think he knew a little anyway about how Caspian came to his throne and him seeing him an old man and all, Caspian was foremost in his mind.

That's right I forgot about that line in the vdt book. But when Eustace is talking to Rillian in LB he says that the friends of Narnia all get together and talk about it. i think I was thinking more of that when I assumed that Eustace knew all about what had happened to Edmund.

Hmm, good point. It's hard to tell when the first time Eustace heard about it was...
I think Eustace might've been told between SC and LB, or wasn't told at all. I mean, it's another person's story, and Lewis is constantly leaving us guessing about other characters' stories.