Poirot gave two explanations first, a mysterious figure who came aboard in shadow and left the same way - committing the crime in disguise. and the 2nd which implicated the other 12 who road the...
All 12 of them do because he was the killer of daisy Armstrong and that were all very close to the Armstrong family so they took it onto themselves to solve it and stab him.
Yes, they do. You have to open a Certificate of Deposit and pledge it as collateral. The limit on the credit card is equal to the amount of the CD you open.