You are correct, I just got ths reply from a pal at HAL.
If someone were to miss the ship in Seattle, the ship’s staff would pack up the guest’s belongings for shipment home at guest expense, offloading it in Vancouver… and the guest’s onboard account gets assessed whatever Customs violation fees that are incurred to Holland America Line by the authorities, for leaving someone behind in a port where it’s not allowed. Even accidental, it’s considered an unallowed debark. No idea on those fee amount estimates, I’m afraid. Could depend on the person’s situation, nationality, etc.
I live just outside Seattle and am looking at a last minute cruise this weekend. It would work out better all around to be able to get off in Seattle but it's looking like we'll be able to swing it.
Never booked last minute, will be exciting if it works out. This for the 4/26 sailing of the Oosterdam.