I agree that there was a collusion between them, but I think it goes beyond just trying to get Lady Middleton's favour. The remarks you point out where Lucy 'chastens' Nancy, seem to me to be evidence of Lucy coaching her older sister, and reminding her what they had agreed they would not say or discuss and how they had already planned to get the information they wanted out of Elinor.

In addition to the examples you cite, there are these:

"But why should you think," said Lucy, looking ashamed of her sister, "that there are not as many genteel young men in Devonshire as Sussex?"

and

"Norland is a prodigious beautiful place, is not it?" added Miss Steele.

"We have heard Sir John admire it excessively," said Lucy, who seemed to think some apology necessary for the freedom of her sister.

I think this is Lucy correcting the blunder that her (not too bright) sister just made. We know Sir John visited the family at Stanhill and not since. I don't think there is any indication that he ever came to Norland, so it's possible that Lucy is lying there and the praise they heard of Norland was from Edward. I think that before they came, Lucy had spelled out very carefully to Anne exactly what kinds of topics they should bring up and which ones they needed to avoid until the time was right.

As for securing Edward, I was thinking that some of the remarks about the Palmers in the previous chapters provide a little hint to how that happened. Lucy is unquestionably attractive. Sir John says she is 'monstrous pretty' and I don't think it's a coincidence that he used the idential expression to describe Marianne, whom we know to be a beautiful girl.

Also, with respect to the Palmers, Elinor thought "that through some unaccountable bias in favour of beauty, he was the husband of a very silly woman ". If you substitute the word 'silly' for other undesirable qualities like 'conniving' or 'vulgar' etc., I think it still holds true. After all, this type of a bias is, as Elinor muses, 'a common kind of blunder' and it does account for "the strange unsuitableness which often existed between husband and wife"--not just the Palmers, but other husbands and wives, too.