I can't answer your STT questions, but I can tell you that Courtesy Rental is about 1.5 blocks from the ferry dock. From the dock walk straight ahead, past the park, the bank, and Woody's; Courtesy is immediately past the police station. It is walkable with luggage as long as you have a manageable amount, otherwise there are always taxis waiting at the end of the dock for ferry passengers to arrive.

Regarding the car rental on St. John - the thing we usually do is leave one person standing at the dock with luggage, and the other goes to get the car. Then you can just drive back to the dock and pick up the luggage. That way you don't have to drag your luggage through town.

Ken

We do the same thing; traveling with toddlers, we NEVER have an easily manageable amount of luggage.

Sapphire is very close to Red Hook and there are always cabs at Sapphire. Don't cut it to close time wise though because sometimes the cabbies wait for additional fares.
The term "island time" applies to almost everything down there and the ferry schedule is more a suggestion than a real schedule.
There is a market at Red Hook just across the street from the ferry and when you return you can pick up supplies and take a cab back to Sapphire.

I've found the ferries to be very much on schedule. Sometimes when docking however, people become impatient and stand up before the ferry has stopped moving which causes the ferry to pull away from the dock until everyone is re-seated. Also, no one will be at the ticket window until about 15 minutes before the hour (the ferries run on the hour after the first two of the morning) so there's no point in standing and looking wih cupped hand around the eyes into the booth looking for someone to sell you a ticket. There's no sign or anything indicating when the booth will be open which if there were a sign would help with the slight confusion which generally occurs. Fares are now (and have been for awhile) four dollars each way per person and a dollar per piece of luggage.