I have no problem with chair saving on Castaway Cay. It’s completely different than on the ship. There are so many chairs. If your whole party is doing the 5K, go ahead and save chairs before the run. If the whole party isn’t running, just have the other people save your chairs so you don’t have to worry about it ... they can get off the ship at essentially the same time.
If you’re going to spend the entire day on the island, it makes no sense to me to NOT claim a spot right when you get off the ship. We always get off the ship and claim a spot as soon as we can and we are never in the spot ALL DAY. Isn’t that how most people do a beach day?? Seriously. We explore the island, ride bikes, eat at Cookies, snorkel, get character photos, try the slides, float on rafts ... are folks actually suggesting that we should pack up all of our stuff and store it (where???) every time we leave our chairs to do something? Do people really do this at the beach?
In my experience, the 5K people don’t really get off any earlier and have no great advantage. On our last cruise, DH and one of my DD’s ran the 5K ... while the other DD and I got off without the runners. We were off the ship at almost the same time.