The boat boys offer services from delivering fresh bread and ice to selling tee shirts and other souvenirs and securing moorings. I certain bays it's best to tie the stern to a tree on shore, we always get one to do that for us. Leaving from St. Vincent you won't encounter the guys selling necklaces and such. They seem to take turns securing a mooring as you enter the harbor in Bequia. There are also boats that will approach you in the harbor with ice, bread, menus, laundry services and so on; a quick no thank you is all it takes and they are on their way.

You should have plenty of time to fulfill your itinerary; we do 10 days R/T from St. Lucia and make it as far as Petit Martinique. The Tobago Cays are a different experience when on a yacht than just making a day trip over. Enjoying fresh lobster and rum punch on the beach as the sun goes down is hard to beat. On Mystique you will be required to pay for 3 days on the mooring, even if you only stay overnight, last I knew it was $80 US.

The Captain will probably welcome your help, even if it's just "pull that piece of string" or steer for a bit. Sailing in itself isn't that hard once somebody sets the sails points you in the right direction. If you get a chance watch the movie Captain Ron.

There is a couple that charter the Grenadines out of Grenada, I believe it's S/V Venda, if I come across it I'll update.
My favorite, Skyelark, is in the Pacific this year and probably won't be back to the Caribbean until winter of 2021 at the earliest.