]I read the phrase “she (Lucy) spoke of her (still Lucy) friendship for them both with the most grateful warmth” as a single thought. After the comma, onto a new thought with: “(Lucy) was ready to own all their obligation (changing to Edward and Lucy), to her (Elinor.)” The vagueness leaves so much open to interpretation.

It does, but I still think that the 'her' in possession of the friendship has to be Elinor rather than Lucy. Lucy would be friends with 'both' of whom? I think it must be read that 'Lucy spoke of Elinor's friendship for both herself (Lucy) and Edward.

] But in light of this: “if he had talked more of one lady than another, or seemed in any respect less happy at Longstaple than he used to be.(snip)
] I’m quite tempted to think that she is still highly jealous of Elinor. His praise of her is obviously unrestrained.

However, when Lucy said that to Elinor, the engagement was still a secret to everyone else. It's different now that the engagement is out in the open, has been acknowledged by Edward and he has refused to give Lucy up despite his family's objections and pressure (which was the reason for keeping it a secret in the first place). No doubt Lucy is still jealous of Edward's regard for Elinor, but I think Lucy feels more secure in the sense that Edward already had the perfect chance to dump her and he didn't.