Well, here's my analysis on the subject:
First off, I hardly think this is anywhere near settled. Expect a sequence of more offers/counteroffers and legal challenges to come. It will be interesting to see this unfold.
Next of all, about AA's veto. AA may have considerable control, but they can't prevent CP shareholders from tendering their stock. Or for that matter they can't stop AC from buying CP stock on the market. In the event AC manages to gain control of CP (or more precisely the holding company, which AA does not own stock in), I expect there would be negotiations, legal manoevres, and some changes to AC's offer to appease AA. Specifically, AC might change the code-share it proposes for CP to be with AA rather than Delta. Also, AC could offer to buy some services from Sabre in exchange for AA co-operating. The bottom line is that in the event of the Onex bid winning (I think it is unlikely), AA would win big. In the event of AC winning, AA could be shut out of large parts of the Canadian market. In the end, AA continuing to code-share with CP would largely be a draw, or a slight loss for AA, but better than than nothing. Their ownership in CP may be to some extent a trump card, but if they can't play it, then they lose.