I know the feeling of being anxious to leave - we're headed to CSA on Saturday!
1)Based on my understanding, this is kind of a gray area. Jamaica operates on a 110V/50Hz electrical system, whereas the US uses 120V/60Hz. Basically, your electronics from the States should work, but I probably wouldn't hook anything super valuable up just to be on the safe side. For what it's worth, we hooked up my wife's laptop last time and didn't have any problems.
2)I haven't heard of in-room DVD players at any Couples resort.
3)It depends on how full the resort is - I would just suggest talking to the front desk when you check in. If they have some open rooms, they might be able to offer an upgrade. I'm not familiar with the best GVS locations, though...
4)You can book a YS Falls excursion through the concierge desk. However, if you're wanting to combine ziplining and the Pelican Bar, you will probably have to use an outside tour company. There are a few out there, I've heard good things about JuJu Tours but can't vouch personally.
Have fun and enjoy your trip!!
Be a traveler, not a tourist.