Not so the Miss Steeles. They came from Exeter, well provided with admiration for the use of Sir John Middleton, his family, and all his relations, and no niggardly proportion was now dealt out to his fair cousins, whom they declared to be the most beautiful, elegant, accomplished and agreeable girls they had ever beheld, and with whom they were particularly anxious to be better acquainted. (Chapter 21)

Elinor principally attributed that preference of herself which soon became evident in the manners of both, but especially of Lucy, who missed no opportunity of engaging her in conversation, or of striving to improve their acquaintance by an easy and frank communication of her sentiments. (Chapter 22)

I do see Lucy as a woman with a purpose. The only reason to manipulate people is to ensure the outcome you desire. Lucy comes to Barton and purposely seeks out Elinor as a particular acquaintance. Her actions are intentional and with a goal IMO. Lucy wants to find out how Elinor feels about Edward, ensure Elinor knows she has a prior claim, use Elinor to recon information about Edward’s family and forward her marriage if Elinor is inclined or foolish enough to be used—such as when she asked Elinor to petition John for a living for Edward in Chapter 24. I think Lucy is uneducated which accounts for her poor grammar and her lack of knowledge but it is not the same as being unintelligent and I do not think she is clueless when it comes to propriety because she is too aware of her indiscretions; I think Nancy might actually be clueless however. (;D)

and the flattery which Lucy was proud to think of and administer at other times, she feared they would despise her for offering. (Chapter 36)

In Chapter 36 Lucy modifies her flattering manipulating behavior towards Lady Middleton because she fears Elinor will despise her for it. I think the fact Lucy modifies her normal behavior so Elinor will not see it shows she is conscious that what she is doing is wrong.

I cannot bear to have you think me impertinently curious…I am sorry you do not happen to know Mrs. Ferrars." (Chapter 22)

When she initially questions Elinor about Edward’s mother in Chapter 22 she makes a point of telling Elinor not to think she is impertinent—this is not something she would do if she was unaware of being impertinent IMO.

I hope my dear Miss Dashwood will excuse the liberty I take of writing to her… (Chapter 38)

When she writes to Elinor in Chapter 38, she asks to be excused for taking the liberty of writing and again she would not ask to be excused if she did not know she would be thought impertinent. Lucy, I think, just does not care—she has her agenda and she will carry it out regardless of what she needs to do. I think Lucy explains her actions very well to Elinor although she pretends this is something she would do if needed although it sounds suspiciously like what she has done since she came on scene at Barton:

Lucy went on. "I am rather of a jealous temper, too, by nature, and from our different situations in life, from his being so much more in the world than me, and our continual separation, I was enough inclined for suspicion, to have found out the truth in an instant, if there had been the slightest alteration in his behaviour to me when we met, or any lowness of spirits that I could not account for, or if he had talked more of one lady than another, or seemed in any respect less happy at Longstaple than he used to be. I do not mean to say that I am particularly observant or quick-sighted in general, but in such a case I am sure I could not be deceived." (Chapter 24)