Conservatives tanking Miers on ideological grounds (or lack of paper trail ground) if she does well at the hearings from a competence standpoint, will make an O'Connell confirmation to be somewhere between very difficult to impossible. They would then have the cover to do it (the right wingers reject a competent choice because she lacks a paper trail, so we can reject a competent choice on the same grounds, with more of a paper trail.) The Dems and Rino's won't stand for it. If I thought otherwise, and thought Bush would indeed select McConnell (which would require getting over the woman bit), that would be a most excellent, indeed compelling, reason to knife Miers. I would join in the crowd around Brutus myself.
My personal view is that if she does well at the hearings, she will almost certainly be confirmed, in which case I agree with your analysis. But, if she in fact doesn't do well at the hearings, then what?