If all liars had an equal facility with this kind of fake emotion, would philosophers be justified in saying that it is nevertheless LOGICALLY POSSIBLE for there to be all the outward signs of innocence, but with none of the internal innocence?

Logically possible, yes. But in practice, you could not use outward signs of emotion to determine whether anyone was lying. If, somehow, there were no other ways to determine whether people other than yourself were lying (preposterous, yes, but bear with my thought experiment for a moment) -- then the best you could do is to say, "well, I know that I sometimes lie, but everyone else has no capacity for lies at all, as far as I can ever know"). In other words, you'd have arrived at a sort of deception-solipsism. Would you agree ?