Latest (and final) update. Starwood prompted me to call the hotel directly. I called them and spoke to an incredibly helpful agent. She said it would be no problem shortening the reservation. She had to cancel the original and rebook, so a short 3-way call with Starwood was necessary to "force" the point redeposit.
The point cost dropped from 54,000 to 40,500, which is what I would expect from losing the 5th night free.
Pro tip -- don't be afraid calling the Swan and Dolphin directly, as they have some of the friendliest, knowledgeable agents I've talked to in recent history.