To address the Satoshi wealth aspect of your question: We are assuming this is a secret message. Of all the crypto friends Charlie has the most secret is Satoshi Nakamoto. A little later Newsweek propped up Dorian Satoshi as the likely Nakamoto. He's in California. No cigar. At the same time fake Satoshi Nakamoto props up on Twitter. Uses same kinds of words in his bio (i.e. Pasquinade. He changes it regularly by the way). None of this helps elucidate the letter. Since then Craig Wright, seemingly a prankster, comes along. He is something else. Could he have pranked Charlie? He's not in California though.
Also, bear in mind that people were told ahead of time that Charlie would be arrested because there was press at JFK when he came home from Amsterdam. The letter seems to have been typed in 1 minute and mailed right then and there for it to show up magically in Brooklyn so fast... Or it was written with much advance knowledge of Charlie's unexpected arrest. It's really weird.