That's the clearest answer I could've asked for, thank you - so the order is: audit is the thing that actually counts, open source is necessary but not sufficient on its own. That reframes my priorities, and honestly it's a bit sobering in a good way.
And I really appreciate you sharing the course takeaway, because that's the part that lands. "There are many subtle ways to get it wrong even with a trusted library" is exactly the fear I should have and sometimes talk myself out of. The fact that someone who actually studied this concluded "don't do it solo for serious use without expert review" is worth more to me than any feature I could add.
So I'm taking this as: keep building and learning, be honest that it's not for high-stakes use until it's been properly reviewed, and treat the audit as the real gate rather than a nice-to-have. I'd rather say that out loud than oversell it to someone who actually needs the protection.
And thanks for the XMPP explanation - the email-style federated ID is genuinely elegant for the no-phone-number problem. Going to study how OMEMO handles the key exchange on top of that.