what are the odds of Shaq making two clutch free throws and Nowitzki missing one to get the Heat a win?

About 800 Billion to one.

Well, mathematically, Shaq is a 46.9% free throw shooter. The odds then of hitting 2 consecutive free throws is:

.469 x .469 = .21 or 21%

However, that's his regular season free throw shooting %.

In the finals, he had been shooting 12.5% from the line. So the odds then of him hitting 2 consecutive free throws last night was:

.125 x .125 = .01 or 1%

1% chance of that happening.

And on top of that, the odds of Nowitzki missing that 2nd free throw was very low, he's one of the best in basketball.