So I finally got a response from Run Disney. Here is what they said
"The Disney Castaway Cay Challenge is only following the 2016 Walt Disney World
Marathon Weekend.
Thank you,
runDisney"
I was surprised at how short it was lol
The rundisney rep at the Rock'nRoll Seattle expo was firm about the challenge not being at Princess also.
Me being me I would still wait to cancel the cruise (might book royal while keeping Disney), just in case, but I'm weird like that.
I won't try! I just don't want you to regret anything.
Royal is awesome. What ship are you thinking about?
Neat itinerary. Be aware that Coco is a tender port and (just like DCL can't always dock at Castaway) sometimes the weather/water just doesn't allow for people to get off the ship. If you do get to go to the island, enjoy the heck out of it. We really liked it. I liked Key West, what little I saw of it!