In constructions like Not X, still less Y, the still less element is effectively an intensifier of negation, equivalent to Not X, and definitely not Y.
So in OP's context, the "literal" meaning would really be something like...
She didn't manifest emotion or contrition even once, and she manifested fear even less often than that.
Obviously in OP's exact context that literal interpretation could be seen as rather odd, since not once means never, and it's a bit nonsensical to talk about something happening less often than never.
In practice therefore, it often makes more sense to understand the intensified negation as applying more directly to Y - some "negating" statement has been made asserting that X didn't happen, was unlikely, or isn't a suitable word for the context, and that statement would be even more true if we substituted Y instead of X.
A common alternative that could usually be used with exactly the same meaning is...
Not once did she manifest the least emotion or contrition, let alone fear
...where it would make more sense to interpret the "intensified negation" as implying something like the preceding assertion would be even more true if we substituted fear instead of emotion or contrition.