My answers:
1) I think it would be an extremely tough fight against McCain. Probably a few people who ran in the real world wouldn't have run against Cheney. However, despite the fact that the Bush administration was toxic among the broader electorate, it was still seen fairly positively by the Republican base. Because of this, Cheney manages to scrape out a hard fought victory against "RINO McCain".
2) If Obama still won the nomination, there was simply a floor that Cheney wouldn't fall below due to Obama's skin color. So probably something like this:
Obama - 420
Cheney - 118
However, if Hillary or Edwards (minus Rielle Hunter) would the nomination, it could've been close to a 50 state sweep:
White Dem - 503
Cheney - 35