To explain why even Marianne might be described as "crying" this and that, you said she does so in moments of stress, exaltation, grief, etc

So that fits very well with how one imagines Lucy might feel. Worried that she could lose Edward to a superior woman she is never at ease. With nerves highstrung she's as eager to please lady Middleton as to lessen the impact of Miss Steele's unguarded revelations.
Calculatingly setting her evil plan in action to make Miss Dashwood aware of her inferior claims on Mr Ferrars. I think it very likely if her voice is shrill.