The final word on Mrs Clay and Mr Elliot is this:

"Mrs. Clay's affections had overpowered her interest, and she had sacrificed, for the young man's sake, the possibility of scheming longer for Sir Walter. She has abilities, however, as well as affections; and it is now a doubtful point whether his cunning, or hers, may finally carry the day; whether, after preventing her from being the wife of Sir Walter, he may not be wheedled and caressed at last into making her the wife of Sir William." (ch 24)

So, what do you think? Who will prevail? Will Mrs Clay "wheedle and caress" Mr Elliot into marrying her? I tend to think not. He has already set her up as his mistress - she is "established under his protection in London". We know she is clever and cunning, but I think that Mr Elliot's self-interest outweighs her cleverness. He nas never done anything, as far as we know, that did not benefit himself. And, as Mrs Clay is now a "kept woman" she has no social status, she is outcast from all good society, and that is important to Mr Elliot. I can't see him abandoning his singlemindedness now, no matter how good she is.