I think Lucy is quite capable of marking her territory with the ring just as you suggest. She was careful to have Edward’s portrait and his letter to oh-so-causally waive in front of Elinor as proof of the engagement. The entire reveal episode seems planned and scripted. Lucy tries to create an opening to confess by asking impertinently about Mrs. Ferrars but Elinor does not take the bait and ask the reason for such a question. Lucy has to bring up her own impertinence again to finally set Elinor up for the big reveal.

Elinor was soon called to the card-table by the conclusion of the first rubber, and the confidential discourse of the two ladies was therefore at an end, to which both of them submitted without any reluctance, for nothing had been said on either side, to make them dislike each other less than they had done before,,, (Ch. 24)