Success through Failure

[Aratlithiel]

Of course, Elrond didn't put any onus on Sam to bear the Ring, only to help Frodo, so he was meeting his performance criteria as far as that was concerned

Agreed - you're right in that no one ever said to anyone in the Company that if Frodo died they were to choose one to carry on so who knows? But it would seem to me that once Sam took the Ring from Frodo, his commitment to fulfilling the Quest was implicit.