I too feel some sympathy for Mrs. Clay, and at this point we have no real proof of ill deeds or ill intentions. As you say, she is only being pleasing in an attempt to give herself (and possibly children) a better chance for a decent life. Who can blame her for that?

Mrs. Clay is also not shown to have particular virtues that would entitle her to a superior match, at least not at this point. We know very little about her.

We are clearly shown that Sir Walter is not much of a catch for anyone. The comment about his unreasonable applications for marriage to women after Lady Elliot's death lead me to believe he was trying to attach women of higher rank than himself. He certainly has no virtue (money, character, rank) that entitles him to that! If Mrs. Clay gets him, would that really be a win?

I think JA was a meritocrat and believed that advancement in life should be due to your personal talents and efforts and not who you were born.