You have to go thru immigration and get your luggage.
Not after an inter-island flight. We just need to get our luggage and a taxi.
You should. 15 minute drive. No reason to purchase ferry tickets in advance.
You should be OK as it will only take a few minutes to get your luggage and get a taxi. It's a short distance from the luggage claim area to the taxi's stand. The ride downtown to the ferry dock takes about 20 minutes. If you miss the first ferry put your luggage in the carrier that the load on the ferry with a forklift and then go upstairs at the ferry terminal and have a cold beer or a snack until the next ferry.
When are you going?