Questions about Andy's testimony

Im watching Andy Cano's testimony in the first trial. I have two questions about it in case someone can answer:

1. I have read here that Andy lied about not having been told by Erik about his involvements in the murders while they were fliying to LA. Does anyone knows how and when is the fact that he lied revealed?

2. Im just wondering (and I know this has already been asked many times in this subreddit but I cant figure out a satisfying answer) how can the letter used in the habeas be real if he testifies to not having heard or asked anything about the abuse after a certain age. Is there any info about it? Has Robert Rand ever explained it?