Extremely Sad and Incredibly Depressing discussion

I am assuming Oskar is telling his story in retrospect and he found out that the renter was his grandfather?

Were all the chapter's that the Grandmother narrated letters to Oskar?

Hi Corey, yes, Oskar knew it was his grandfather - he said so in the last chapter. I don't think he explains exactly how or when he realized it but he definitely knew at the end - so it was a retrospective story, yes.

And yes, the grandmother was writing to Oskar in all those letters.