For the second question, your answer is correct. The encryption keys derived from the double ratchet are deleted once used, and users can set to automatically delete messages that have been read. Therefore, a hacker can only access the most recent messages and cannot retrieve past messages.

When I said that deploying the double ratchet wouldn't be more difficult, I meant that we can directly reuse the open-source code, libsignal.

If one party in a chat sends multiple messages in succession, the symmetric ratchet will come into play. Moreover, the double ratchet algorithm maximizes the use of the DH ratchet, with the DH ratchet turning each time the chatting parties reply to each other, instead of changing the DH at set intervals.