I think we've reached a point where not trusting Lucy can be taken as a given. So what can be the meaning of this?

"So far was she, at the same time, from any backwardness to give Elinor that credit which Edward would give her, that she spoke of her friendship for them both with the most grateful warmth, was ready to own all their obligation to her, and openly declared that no exertion for their good on Miss Dashwood's part, either present or future, would ever surprise her, for she believed her capable of doing anything in the world for those she really valued."

Jane's brevity and succinct style are well known. So when confronted with a narrative sentence about the feelings of a woman such as Lucy that runs on in this manner, it requires a great deal of parsing and analysis.

On it's surface it looks like Lucy is grateful for Elinor's help in forwarding Lucy's & Edward's interests. Still, two segments in particular strike me as subverting the whole. "...friendship for them both" seems to mean, (to me, considering all that's gone on before), that the friendship would not extend to Elinor by herself. The grouping of Elinor and Brandon almost purposely eliminates the need to see the individuality of either or both of them.

The other phrase that seems to put a qualifier on the entire sentence is the last: "for those she really valued." Lucy knows full well that Elinor does not truly value her. She also suspects that Elinor does truly value Edward. Knowing this, "she believed her capable of doing anything in the world" no longer looks like praise. There's almost a sense that Lucy expects Elinor to try to win Edward.

There's a lot more in this sentence that cannot be as it seems. The beginning phrases, "So far was she, at the same time, from any backwardness to give Elinor that credit" have a certain double talk nature to them that I can't, at the moment, explain with clarity.