it was lucky guesswork. When information is presented piecemeal, sometimes the visual details lend themselves to solving the riddle. For example,

When the first evidence of the old man having one leg was presented, I thought, "ah, I'll bet it was bitten off by a shark!"

When it was made clear that Walpole was on the raft, I thought, "He is somehow in on this."

When he was awarded one million from the will, it all made sense.

The plot twist was terrific, and it was far from obvious at the beginning. But again, once the pieces trickle in, the paths to solution begin to lessen.

By the way, fabulous observation about the Biblical references. I didn't notice them readily until you made mention of them. Something I will start listen for in future shows!