I agree with Barbara B on the Oscar. ;D

Ignorant—lacking in knowledge or training; unlearned, lacking knowledge or information as to a particular subject for fact, uninformed, unaware; due to or showing lack of knowledge or training. (dictionary.com)

Illiterate—unable to read and write, having or demonstrating very little or no education, showing lack of culture, esp. in language and literature, displaying a marked lack of knowledge in a particular field. (dictionary.com)

You raise some interesting ideas; I will try to address some of them. There are two parts to Elinor’s opinion, as I see it, that you claim to be unwarranted, first that on Lucy’s actual abilities and second on her behavior to Lady Middleton and her children. I believe Elinor spent enough half hours with Lucy to evaluate the quality of her information and how she uses it in conservation. I think “want of information in the most common particulars” is an indication of a lack of education, ignorance and illiteracy—it is unfortunate JA did not describe the conservations in which Lucy tries to conceal that she does not know facts and information pertinent to the topic which leads to this opinion of her. I also do not see ignorant and illiterate as particularly mean spirited judgments as ignorant and illiterate both describe lacks in knowledge not a lack of intelligence. Of the definitions above probably the only one Elinor does not mean to imply IMO is that of being unable to read and write. As to Lucy’s actions, I think three before Chapter 22 is pertinent to Elinor’s feelings and perhaps to her opinion about Lucy in Chapter 22.

1—I think when Lucy denies knowing Edward very well in Chapter 21 she raises protective feelings in Elinor; Anne lets it slip that they know Edward very well and Lucy immediately claims the reverse. Lucy’s reticence to admit to the dept of the acquaintance makes Elinor think Lucy knows something to Edward’s disadvantage. Elinor’s thoughts are protective of Edward—I think this is an indication of how much she does esteem him and I am sure Lucy’s denial is something which does not endear her to Elinor. Of course Lucy does know something to Edward’s disadvantage as both Elinor and the reader find out when Lucy reveals her engagement which necessarily shows that Lucy is lying about her relationship with him in Chapter 21.

2—Lucy’s insincere attentions to Lady Middleton and her children reveal a dishonest streak in Lucy. I think the observation of a person frequently being insincere to another (the buttering up of LM as you put it) does indeed devalue that persons attentions to yourself. How is Elinor to understand Lucy’s attentions to LM otherwise? I see you do not suggest that Lucy’s attentions to LM and her children are sincere and how can her behavior towards them but reflect a sense of dishonesty in Lucy’s dealing with people. Like Elinor I would take that persons discourse with a grain of salt.

3—in the Chapter 21, Elinor does not desire to know the Steele sisters further but the narrator says they feel just the opposite and pursue a relationship with her and it is again mentioned in Chapter 22. After Elinor is more or less forced into a closer acquaintance with Lucy she does admit to her good qualities and even admits to herself that she can be agreeable company, “Lucy was naturally clever; her remarks were often just and amusing; and as a companion for half an hour Elinor frequently found her agreeable”—Chapter 22. I am not sure what else you can want from Elinor; Lucy receives her pity for wasted talents but Lucy is not entitled to be liked and she is treated with civility by Elinor despite the fact their relationship is made closer not because she wants it but because she is pursued by Lucy. Sir John just pushes them together, Lucy is the one who oversteps Elinor’s bounds—I think Elinor has a right to set boundaries and decide whether a person is an agreeable companion.

IMO there is no evidence on which to base a conjecture that Elinor "feels she cannot compete in the looks department with Lucy" and “she must chop her down to size by noting her lack of education and good grammar” because Elinor does not know Lucy is engaged to Edward when this opinion is given in Chapter 22 and therefore has no reason to envy her for it. While Elinor is not described as beautiful as is Marianne she is also not described as being ugly or plain—“Miss Dashwood had a delicate complexion, regular features, and a remarkably pretty figure”—Chapter 10. There is also no indication that Elinor places an extraordinary amount of emphasis on beauty—she has fallen in love with Edward and “his person can hardly be called handsome”—Chapter 4. On the other hand Lady Middleton’s “elegance of appearance”—Chapter 6 does not ingratiate her cold reserve to Elinor either; in fact Elinor shows no bias towards outward appearance either way IMO.