OS X Filevault Login issue
How did you check which account can decrypt the disk?

I suspect (WAG) what happened is that you set up your new account so that it duplicates the UID of the previous admin account, so it shows up in one place as being authorized, but is not actually authorized. You removed account IS authorized for file vault, even thought eh account no longer exists on the unix-y side of things.

I'd try all this again, making sure to add the new accounts with new UIDs and that you add them to the FileVault list.