8. dec is fine so long as you complete travel by dec 15th when rates skyrocket and many places require 2 wk stay if going for xmas
4. it is very easy to hop between st. thomas & st. john. 20 or 45 mins ferry ride depending on which st. thomas harbor you use (red hook or charlotte amalie)
2. st. john & st. croix are quite safe. st. thomas somewhat less so particularly if you're going at out night.

you need to tell us your budget in order for us to make any useful suggestions re where to stay/what to do. on st. john many/most people rent villas. would this appeal to your group? you would need to rent a car/jeep. how many are going?

I disagree with Virgina that St. Thomas is safe if we are are considering after dark down town. If you are staying at your ghotel or taking a taxi from your hotel to another and back that is fine. St. Croix is has issues but not to the extent of St. Thomas but so does NYC. St. John on the other hand might be a wiser choice and if you rented a 4 bedroom villa for about $4000-$6000/week and did not stay beyond Dec.15-19 period you could get a nice villas and rent a van for all or part of your stay. In the town you will find a number of small restaurants and bars. Snorkeling is far better off St. John than than St. Thomas or St. Croix, in my opinion. Island hopping is possible from St. Thomas but a bit costly. St. John is a lush island yet has a number of lovely beaches.Hope this helps.

Island hopping from St Thomas is a breeze! St John is a 15 minute ferry ride away 46 each way. Tortola and Jost Van Dyke are about 45 minutes by ferry (need a passport for those), or you can see lots more of the British Virgin Islands on day charters for $150 or so (includes lunch and drinks).

December before Christmas is great. After Christmas, it gets crowded and expensive. And cold!