Thanks all! I didn't know about the Marriott tip for getting $1000 off.
Here's what I'm thinking so far: Use the 2 browser trick to get them 2 Citi ThankYou Premiers each for a total of 105,000 points (incl spend) per person x 2 = 210,000 or $2100 in travel credit for a cruise booked through Citi's affiliated travel center. That $2100 should pretty much cover the cruise, or darn close.
Then get them to sign up for United cards to cover the flights. They can either pay for hotels out of pocket, or maybe I can work on getting them some hotel points too.