The ending was definitely a bit over the top, but it contains that
iconic line “I drink your milkshake” which ensures it will be
remembered for quite some time. I also loved how it just abruptly ends.

As for your questions, I think the brother was a casting issue —
there was a different actor playing the preacher who dropped out of
the film, and Paul Dano, who was originally playing the brother,
changed roles.

I don’t know if there was anything specific that tipped off Daniel.
That might be a plot oversight. But it seems the character was so
paranoid and isolated from everyone around him, it makes some sense
that he would eventually question his own brother. After all, he does
the same to his own son.

I found the score to be too heavy-handed in parts, adding unwanted emphasis to the snails pace of many, many of the scenes. For me, it was far too much like the “2001” score in several sections. The ending was maybe suitable but also kind of unsatisfying.

The biggest stumper — why did we meet the brother Paul at the beginning only for him to never reappear throughout the remainder of the film? I kept waiting for that shoe to drop. Was that some kind of message? And I didn’t catch what tipped Daniel off during the beach scene that Charley was not the real deal.