I agree with the postulate of Coolidge beating Smith: the 1928 election was accompanied by some of the dirtiest mud-slinging in history. All of the old anti-Catholic whisperings were trotted out and used, especially in the south: otherwise it's highly debatable whether Hoover would have carried Virginia as he did in OTL. I don't doubt that the same would have applied with Coolidge as the GOP nominee.
Now as to a death in office: I suspect that Coolidge's death in early 1933 was hastened by the death of his son rather than the stress of the office. Remember that the 1920s were not very demanding upon the presidency. As such, absent his son's passing, I suspect Coolidge would have lived somewhat longer--say, into the mid-to-late 1930s.
I question that Dawes would have run with Coolidge in 1928: those two had a falling-out over Dawes being absent from the chair in the Senate on a critical vote. He would have been able to cast the tiebreaker in favor of the administration; however, he had been taking a nap at his hotel residence over lunch, and despite a frantic cab ride, couldn't get back in time. That soured the relationship. I suspect that Coolidge might have chosen Charles Curtis instead, as did Hoover. Conceivably Coolidge might have made use of Dawes in some capacity assuming Wall Street crashed as it did in OTL: don't forget Dawes was highly successful in banking and economics, and would have been better suited for the post of Secretary of the Treasury as opposed to being vice president.