Sheridan is clever, demented, focused on his book and his idea of truth about Vietnam, anti-establishment, manipulative, a jerk, smart, and stupid.
So yes is a reasonable possibility to any of your questions about his motivations.
He's also very alone, has no friends, although is a very social person. And he knows he's dying.
Hell, if we're going to psychoanalyze: what are my motivations? EU loves to say he reads people and he knows my motivations exactly!
Could Sheridan be that crazy/clever/conniving? To have passport evidence that he was in Nepal during Norjak and not show it to Fryar?
Maybe he loved the limelight and limbo. If he proved he wasn't Cooper, party over.
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