Ok, that helps. From my understanding, you're wanting to do the group shots outdoors near sunset. Keep in mind that sunset in Jamaca in July will be somewhere in the neighborhood of 6:45pm, so that gives you a an idea of the timeline you'll have to work with.

While a couple of light stands, speedlights, umbrellas and remote triggers may produce pleasing results, there's something else to consider - wind. I now live about 30 minutes from a beach (Tybee Island, GA), and I can say that it's usually very windy there (same was true on my visit to Myrtle Beach a couple of weeks ago). That makes using an umbrella challenging because of its sail-like characteristics. I'm not saying it won't work, but you'll likely need a) a lot of weight holding your light stands down or b) at least two helpers to keep the light stands from tipping over (an issue if everyone needs to be in one of the shots). Another concern is that if you stack people in rows, the people in the back may be shaded unless your lights are positioned relatively high. And it gets tough to pack light stand that can reach a decent height while remaining airline pack-able.

You may want to invest in some colored gels, too. By using a CTO (or 1/2, or 1/4 CTO) to match the ambient sunset (or indoor tungsten for the reception), then the color of the light hitting the subjects will more closely match the ambient. Otherwise, the color hitting the subjects will be very cool/blue in comparison. Also, I've found that for indoor locations (where the ceiling is a relatively normal height), bounce flash works very well. I'd suggest picking up the Rogue Flashbender 2 Small to kick a little light forward while using the ceiling to do the heavy lifting. I like the Flashbender 2 small because it's really easy to pack and it isn't bulky in-use, and you get the benefits of on-camera ETTL for exposure calculation.

Those are just some thoughts off the top of my head.

PS: On the topic of backup solutions, I ran across this interesting article today. I did some investigating, and it looks like a pretty cost-effective solution. Though, you'd likely want to make/design a case for the circuit board.