@TomF with TransactedSession I was rolling back on message 3. I was not restarting the flow. After few re-deliveries message 3 and all messages behind it were put to the DMQ.
However, I successfully implemented my scenario with ClientAck and restarting the flow to simulate “nack”. So I’ll stick to this solution and wait for a Nack() method added by you guys. Regarding the MaxUnackedMessages - do I understand correctly that for my needs I need to set it to 1, so only one message at a time will be delivered to the client?