The specific questions:
You can try elite dry cleaners for your dress, they are next to the rolling pin and tamarind inn on STJ. If not they can point you in the right direction.
The Mixology in the Lumberyard has great prices on liquor and so does the Marina Market. Otherwise if you are looking for fine wine, try Starfish or Simple Feast but obviously you pay for the good stuff.
Entertainers are all over the island, it depends on who you like.
I know in addition to Morgan's (which i think is overpriced), there is beach bar, front yard, Bannana Deck, Chloe and Bernards(fine dining and piano music), Blues bar(Coral Bay), Miss Lucy's(i like brunch there) with all live music it varies so much that you have to call weekly to get the schedules. I would play it by ear and call the places and ask what they have going on the week of your arrival if you are really interested in hitting certain people.
I would take the red hook ferry, it leaves every hour on the hour and if your flight is delayed, you just catch the next boat. The charlotte amalie ferry isnt as frequent and the ferry costs a little extra to negate the shortened taxi ride.