Jean-Marie: The transformation between steps 2 and 3 (1 0 0 -> 1 1 0) is the same as for the one between steps 8 and 9; we are working with the entire stack of 3 coins. The first/top coin exchanges with the last/bottom coin and then all coins change their orientation.

Jean-Marie: I think the phrase, "invert the substack" is the issue. For me, this means to take the substack and invert it as a unit. The coins exchange positions in the substack as well as inverting their values. It is a single physical action that translates into two separate operations on the coins. I may have done better to write, "flip the substack as a unit."

For the bonus question, we can rule out any number that has a prime factor greater than 26 (z). The product found by Jason Friedman is the first number greater than 10^6 that does not have such a prime factor. The product 999856 is the largest number less than 10^6 to be part of a possible solution.