It is rebuilt from LINK Nantucket's IDX feed every time the site's listings refresh, several times a day. Only open houses marked public appear, and each one links to the listing page for that house. Broker-only and office-only events are never shown.
Fewer, but yes. Most Nantucket showings happen by appointment year-round, and brokers schedule public open houses selectively, mostly on summer and shoulder-season weekends. An empty list does not mean nothing is available to see; it means you should ask for a private showing.
No. Anyone can walk in during the posted hours. In Massachusetts the agent hosting will hand you a disclosure form explaining who they represent, which is not a contract. If you are already working with a buyer's agent, say so at the door, and if you would like Sean to preview a house or meet you there, call or text 508-228-4578.