Reasonable doubt means there is reasonable doubt. Or if you prefer metaphors, the term is naked. Doubt, that the progenitor of the pass phrase was likely the one who transferred the coins to another address(s) is sufficient to preclude it as a determining factor. That is, of course, if your lawyer is good enough to be persuasive.
Meanwhile, people have been convicted with far more implausible evidence (eyewitness testimony comes to mind) so this will be the least of your worries.
You tried to infuse more meaning to that term by trying to add to its definition a-thing-that-it-isn't.
>'Reasonable doubt' does not mean 'no matter how improbable,'
You're right. "Reasonable doubt" also does not mean "Ice Cream Sunday", but that doesn't make it "Pudding" instead. The term means exactly what it says, so I say again, "Reasonable doubt means there is reasonable doubt".
It's times like this I genuinely wish HN had an "ignore" feature since I don't believe in down votes.
Meanwhile, people have been convicted with far more implausible evidence (eyewitness testimony comes to mind) so this will be the least of your worries.