Let's look at the question of whether Dustin/Kandice get left behind with a bit more objectivity, specifically from the perspective of flights. As long as they can make the 1810 flight from Lanai to Honolulu, it doesn't matter if they are 2.5 hours behind the other 2 teams. Any team getting on the 1800, 1805 or 1810 flights out of Lanai will make UA76 out of HNL to SFO departing at 2105 and arriving 0501. World Race Productions did this for sure - scheduled tasks on Lanai to assure that no team would reach Lanai airport in time to make the 1540 flight but all teams should make 1800 to 1810 departures unless they badly screw up. This ensures that the WRP favorite technique of BUNCHING prevails and no team is left behind.
The probablity that Dustin/Kandice lose a huge enough amount of time in a kayak spill is quite low. Even if they do, they have other flights out of Lanai with connections in Honolulu that will put them one to two hours behind but not totally out depending on how diffidult the MEMORY challenge is.