Raulo, you have a mathematical error on page 1, in equation (4).
Your \xi depends on n. Consequently, you cannot pass to limit only in part of the expression.
The expression q:=1- (1/2^32* D) is a real number < 1. Consequently, Q(n)=q^n -->0 as n --> \infty.
This makes perfect sense, because the probability that you will "almost surely" (in the sense of probability theory) find a block if you wait long enough (infinitely long).
It's not an error, though tricky and somewhat unclear.
He is assuming that \xi is constant while n and D are variable. For \xi=2, for example, he is considering a case that the hashes calculated are twice the average needed; he then considers what happens when D, and correspondingly n, go to infinity (continuous case). In this case the chance of not finding a block is indeed exp(-2).