I also welcome you to the board and it sounds like you've really done your homework. If I read your timeline right, you will arrive in Negril on a Sat - so have Sat night, Sunday, and Sunday night there before leaving to St. Elizabeth on Monday.
I'm sure you've read posts that Sat night party is at Seastar Inn - with great buffet, band, and african drummers. It's a great deal, the staff is great, and that would be my suggestion. It's even within walking distance of Rick's, so you could check out sunset at Rick's and just have a drink or two, then walk over to Seastar for dinner buffet and close out the night. Bring your suits, because you'll be allowed to use the pool at Seastar, and by then you'll probably be ready for a dip to cool down between sunset and the start of the party. They have a free shuttle that will take you back to your hotel, but be sure to tip the driver. Even though you have use of a car, I would leave it for this - unless your Jamaican friend wants to be a designated driver.
Sunday you will probably just want to relax at your hotel and explore the surrounding beach area. For Sunday night there are 2 choices - either the beach party at Alfred's (they have great dinners there, too, at great prices) - or the Sunday night beach party at the park by the craft market. Alfred's will have a reggae band and a pretty much 50-50 mix of locals-tourists, while the party at the beach park will be dancehall music and pretty much 90-10 locals-tourists. Being with your friend, you will not have a problem at either place. There will be vendors selling some food at the beach park, as well.
To answer your question about money - either currency is accepted, but you will get better deals if you use Jamaican currency. With dollars, they will still give you change in Jamaican - and the merchant can set whatever rate he wants. I'd suggest using just Jamaican also so that you minimize having to calculate the exchange for both purchases and change. If you've read alot of these posts, I don't need to tell you to exchange money at the cambios or banks - not airport, hotels, restaurants or stores - to get best rates. Make sure to have many small bills on hand - especially once you get out of Negril - smaller places have hard time making change for anything over $500J.
Couple of other tips - you said even the obvious in case you forgot:
Bring SPF30 or higher lotion with you - it's expensive on the island - and be sure to use it. Nothing worse than ruining a planned active vacation by getting burned and having to hide from the sun.
If you haven't already - search the posts for "sand fleas" - so can avoid them at dawn/dusk on the beach.
For your camera - bring plenty of sim cards (again about triple price on the island). Also, I always have 2 spare batteries. Nothing worse than filling all your sim cards, or losing your charge in middle of an activity. You can probably get them cheap, and in time for trip, on ebay.
Other suggestions for not-to-miss food. I know you're not in Negril long enough to hit them all, but here are suggestions.
In Negril - Bourbon Beach on the beach for jerk chicken cooked traditional way over Pimento wood on a grill, not a barrel. Eddie's De Bar on the West End for jerk chicken cooked over a barrel, Canoe Bar for Happy Hour and appetizers, Red Dragon near town center for lunch for best jerk pork in Negril. Taxi drivers will know them all.
In Treasure Beach - Frenchman's Reef has huge, delicious lobsters at great price, when I was there they even had a 2-for-1 incredible deal.
In Montego Bay - you can't go there without hitting The Pork Pit.
