2

For those saying that the receiver can always screenshot and share in #2, sure. But apps can lead the receiver in the mindset of the sender. If you receive a simple link, without explanations, you will think it's ok to reshare. The link is public after all. But

Correct. You would create #2 events for the past content. To revoke access, we can make the events replaceable so that at any point, you can rotate the secret. You will need to update #1 and all #2 events, though. Those who didn't get the update cannot decrypt