One date field, one source of truth
The event date lives in a single custom value. The registration page reads it, every reminder is offset from it, the replay window closes relative to it, and the waitlist workflow points at the next one. Move the date and nothing else needs touching.
Scarcity that is actually true
The seat counter decrements on each registration rather than running on a script that resets every visit. If you say fifty seats, fifty is what the page enforces, and registration 51 goes to the waitlist. Scarcity works because it is real; fake counters get screenshotted and mocked.
Five days to close, then stop
The replay sequence runs five days and ends. Contacts who did not buy go back to the general list rather than staying in a launch sequence forever, which is what keeps a list usable for the next event.