Maybe this will help narrow your choices.

Barbuda is a sparsely populated island with less than 1500 people in some 62 sq. miles. It is flat, arid and scrubby. The big draw is it's wonderful 11 mile long light pink sand beach that has virtually no one on it. The reset you are considering is relatively small and there's really not much to do on the island except lie on the beach. You can explore the main points of interest (the caves and the Frigate bird sanctuary in less than 1/2 a day. This island does not get many tourists - there probably less than 150 "guest rooms" on the entire island. It's the type of place you go to "get away from it all". Go here only if you want to lie on the beach and you are sure your fiancé won't get easily bored. The Light House is a very nice resort but it is quite remote.

On the other hand, Galley Bay on Antigua is an excellent resort for honeymooners. It's a relatively small but full service resort that offers you the option to relax on an uncrowded beach or participate in various activities such as day sails, snorkel trips, island tours, etc.

Palm Island, in the Grenadines, is a wonderful "private island" resort but once you are there you are "THERE". Other than "on-resort" activities there's nothing to see or explore on the island. You'll have to venture of-island to go shopping, see sights or take part in normal tourist related activities and getting off island is not always easy. Go here if you plan on spending most of your time "on-resort".

Young Island is a small resort on a tiny island in the bay at the tip of St. Vincent. There is a small beach for use by resort guests. It's attractive but I wouldn't call it "beautiful". You have to take a 5 minute boat ride every time you want to leave the island to visit St. Vincent. St. Vincent is a lush island but it has very few beaches.

St. lucia is a large (23 sq. miles) island with towering mountains, lush rain forests, waterfalls and lots of flora and fauna. The island has many visually stunning darker sand (think brown, gray and even black) beaches lined with dense pam tress and other natural vegetation. There's plenty of activities to do and things to see all over the island. From what you describe in your list of 'wants" and the need for activities for your fiancé this may be the perfect island for you. Anse Chastenet is a fabulous resort with fantastic views.

FYI - mosquitoes can be found anywhere in the Caribbean. The number you might encounter is more a function of recent weather conditions than it is at a particular location. You are more likely to encounter mosquitoes after periods of prolonged rains and/or if there are little or no breezes and they are most active at dawn/dusk near standing water and along the beach where the vegetation meets the sand. Avoid those areas at those times. You can lessen your chances of a "close encounter" by avoiding the use of perfumes, colognes, after-shaves and scented hairsprays and powders as the sweet aromas act as an attractant.