Lady Russell falls to the right-minded charms of Mr. Elliot but they are not alone in fueling her liking. The conviction he is interested in Anne brings unexpected relief for as Mr. Elliot “became known to her, she grew more charitable, or more indifferent” (16) to Elizabeth’s and Sir Walter’s favor of Mrs. Clay. Mr. Elliot inspires hope Anne will soon be free of “the partialities and injustice of her father's house” (4) and to sweeten so equal a marriage she will be eventually settled permanently near herself (4) in her dear friend’s place at Kellynch. The savior of Anne cannot but find comfortable room to live in Lady Russell’s heart. Her satisfaction in him outweighed all the plague of Mrs. Clay (16). Mr. Elliot has fortune, independence, the best connections (Sir Walter’s heir) while Frederick “had nothing but himself to recommend him” (4). The differences in situation seem to be the least of the evils. I see a comparison much to the captain’s detriment in the praise of the other. All that her favorite is “without” I think she applied to the captain:

His manners were an immediate recommendation; and on conversing with him she found the solid so fully supporting the superficial, that she…could not seriously picture to herself a more agreeable or estimable man. Every thing united in him: good understanding, correct opinions, knowledge of the world, and a warm heart. He had strong feelings of family attachment and family honour, without pride or weakness; he lived with the liberality of a man of fortune, without display; he judged for himself in every thing essential, without defying public opinion in any point of worldly decorum. He was steady, observant, moderate, candid; never run away with by spirits or by selfishness, which fancied itself strong feeling; and yet with a sensibility to what was amiable and lovely, and a value for all the felicities of domestic life, which characters of fancied enthusiasm and violent agitationseldomreally possess. She was sure that he had not been happy in marriage…but it had been no unhappiness to sour his mind, nor… (16)

I take “the superficial” to mean Mr. Elliot’s agreeable correct manners and “the solid” to be what she apprehends to be his steady principled character. It seems Lady Russell believed Frederick’s superficial, such as it was, was not fully supported by the solid. It is Frederick whose value for rank is tinctured by pride and weakness. I doubt he would grant the Elliot’s cousins at Laura Place to be good company as did her favorite but rather “their being nothing in themselves” (16) as Anne considers them to be. It is Frederick who was wasteful rather than appropriately liberal, spending freely, what had come freely, had realized nothing (4) but a great deal of display. Like Mr. Elliot Frederick judged for himself in everything essential but unlike could defy public opinion on points of worldly decorum—(?)—from what comes next in her praise, I think this means he could be indecorous in expressing his opinions. It is Frederick who was often run away with spirits or selfishness and fancied it strong feeling. It is Frederick whose value for domestic felicity was merely fancied enthusiasm. It is Frederick whose mind was soured by disappointment—his miserable parting with Anne would seem confirmation.

But he [Frederick] was confident that he should soon be rich; full of life and ardour, he knew that he should soon have a ship, and soon be on a station that would lead to every thing he wanted. He had always been lucky; he knew he should be so still. Such confidence, powerful in its own warmth, and bewitching in the wit which often expressed it, must have been enough for Anne; but Lady Russell saw it very differently. His sanguine temper, and fearlessness of mind, operated very differently on her. She saw in it but an aggravation of the evil. It only added a dangerous character to himself. He was brilliant, he was headstrong. Lady Russell had little taste for wit, and of any thing approaching to imprudence a horror. She deprecated the connexion in every light. (4)

I think combining Lady Russell’s opinion of Frederick then (above) and now (Ch. 16) gives us a better idea of why she believed Frederick unworthy and dangerous. I think she saw selfishness Frederick’s attaching and proposing to Anne with “all her claims of birth, beauty, and mind” (4) without fortune or connection and a future charitably described as uncertain, possibly sinking her “into a state of most wearing, anxious, youth-killing dependance” (4). I suspect she viewed Frederick’s certainty in future prosperity, supported only by confidence in his abilities and the fickleness of luck, as worthless, the mere boasts of a prideful man. She had no affection to bind her faith to the insubstantial. I think Lady Russell’s comparisons to Mr. Elliot also suggest she believed Frederick’s feelings for Anne were changeable and thus his attachment unsteady. Rather than deceitful, he was too passionate, too selfish and too unsophisticated to understand the difference between shallow transient fancies and strong steady, lasting feeling. I think she was wrong but if she believed Frederick to be unsteady in his affection for Anne it makes her continual steady and tender advice to end the engagement (4) a great deal more understandable.