Lucy is shrewd enough to hold on to Edward as her social and marriage insurance.
Edward's stuck by his promise to marry her, and he may find a living as a clergyman. Importantly, he's still got £2,000 so Lucy wouldn't want to throw that away. ;)
And as you point out, the formidable Mrs Ferras who holds the pursestrings of the Ferras family may relent somewhat. Unless a better finacial proposal comes by- Lucy is willing to marry Edward. She'd be a gentleman's wife.
In ch. 39, Col. Brandon asks Elinor to tell Edward he wants to offer him the modest living at Delaford worth £200 a-year, though considered a sum barely sufficent for a single professional man.
Lucy is prepared to wait. She has nothing to lose-she and Anne likely have less money than the Dashwood girls; little education or refinement.
Lucy realizes Edward no longer loves her, but her rival Elinor- yet it is not a practical consideration for her.