Ha, actually I didn't know about that until I saw it mentioned on CruiseCritic today. But actually, my issue is resolved now - my wife's and my ecredits (the ones that were open but said their values can't be determined) now show up correctly on our account.

The ecredit for my mother-in-law (the one showing as closed) had its remaining value assigned a new voucher number when I partially used it to book a new ticket. (This was at the bottom of the email with the confirmation; I had missed that bit until now.)