I was hoping that the free shuttle between Trellis and Scrub/Marina might help you out, but with a 9:30 AM departure from EIS, that's a no-go. The free water shuttle doesn't start early enough.
Staying on a ball at Marina Cay for the overnight is doable, you just have to move the boat early AM to Trellis, pick up a ball, and dingy in to the dingy dock with her luggage. There is a dock there for that purpose, as opposed to the Ferry dock for the Scrub/Marina Cay shuttle and the NS Express ferry.
Trellis is secure and easy. I'd take a ball on the outside rather than close in, just to have more air through the boat overnight. Trellis has always been swimmable, the beach is very pleasant with some shade in places, and the shops and food are fun. It is an easy and close walk to EIS, hardly worth a taxi, but you can also arrange that, too.
http://www.bvivacation.com/transportation/tortola-taxi-fares-rates.php#.V1gdNCFX-ic. Given the AM flight time, you can be out of Trellis and on your way to your next destination with the better part of a whole day in front of you.
