For the (off-topic) sync issue, maybe there’s a straightforward approach.
- Author has a toggle that forces sync, on at the start. Readers cannot veer off from author’s POV when this is on.
- Author can turn it off whenever. That mode is the existing behaviour where readers can do their own thing at their own pace (but can sync back whenever).
- If author closes review, it automatically turns off. If the author reconnects, it’s stays off, but author can turn it back on.
Could be an action button in the same spot as the “sync” button on the reader side. Depending on current state, either says “Allow async” (to turn off) or “Sync readers” (to turn on).