Okay, I did get some better answers. The TA at Costco finally had the
DCL CM come on the line to answer my questions.
So, first, apparently all credits have been given already, but if you want the refund they will process it. In that way, there's no time limit, other than I have 15 months (per the offer) to use those credits before they expire.
Each person (
castaway club number) gets their own credit, not per reservation and each can be used on any sailing they want. It doesn't have to be the same sailing, but it is only one sailing.
Any leftover credit can be applied as an OBC or to anything you can book through DCL. So if you book air or insurance or transfers or even pre-pay tips through DCL, it will cover it. She said it will even cover the land portion IF you book that land portion through DCL and not through WDW.
The credit is the percentage promised, less the taxes. The taxes for the cancelled cruise are going to be refunded to each person in the method we paid, so don't toss those gift cards, people.
They have not made a decision about placeholder credits yet. The CMs have been asking the same question and no answer has been determined. Presumably, once that happens, our TAs will be notified. I did ask for a direct email, if it comes to pass, but I don't know if that will happen.
You can not change the name of a credit recipient. The credits are non-transferable. You can put people in rooms however you like, though. Also, one person's credit can't be used to cover someone else, like an extra guest.
Hope that helps anyone else with questions.