1 - I doubt you will be able to make the 4:15. Consider ditching the checked bags, if you can. You probably won't need or use well over half of what you bring. Unless there is some specific piece of equipment or something like that, you probably don't need to check anything. Camera, credit cards, passport, hat, sunglasses, a good pair of flops and a pair of shorts and couple t-shirts are all that's really needed.

2 - Mal's suggestion is a good one. There are obviously several other options in Road Town as well.

3 - My personal recommendation is to hit Guana and Sandy Spit and skip Marina Cay. Given how limited your time is, I would probably recommend that you skip renting a car on Virgin Gorda for this trip. Hit the Baths by boat early morning and enjoy more time in the water and relaxing. Do the car rental and explore Virgin Gorda next trip when hopefully you can have a bit more time.

Whatever you decide, you can't go wrong! The best thing you can do is just try not to plan too much and don't try to cram too much in. If you want a vacation where you feel rushed constantly moving from one thing/place to the next, go to Disney World! There is too much to see in the islands in one trip, even a long one! Slow the pace and enjoy! Excited for you and hope to hear about your trip when you get back!