Hey folks, I spoke to designers Toby McCall and Jesse Sky and this is what they had to say about this:
When Nightmare Mode was released, Overload had no activation time, but its ballistic component was not working (which meant Overload could be completely ignored). We corrected this in the patch. In Nightmare Mode, the optimal path will always appear on either the left or the right wall, so a wall will always be available for tanking the demolitions probes.
Very interesting. While I'm absolutely grateful for the efforts to make this fight harder and more interesting (at least for the non-solver), there are some problems with the way this was done. Specifically, putting the optimal path on *either* the left or the right wall is problematic because you need to commit to one side or another almost immediately. This is particularly true of the right wall, since it is one square longer than the left. If you get unlucky and commit to the wrong side, there's no viable way to backtrack in time to make the enrage. So, the best we can do is just reveal left and right sides as well as the second square in the respective files and make a guess based on which ones turned up green.
What I think would be really nice to see would be if the optimal path were always on the left, but not necessarily a straight line. It is actually possible to defuse *one* mine that is not against the wall, so long as it connects to a chain of greens which lead back to the wall with no defusal. This would be interesting, since it would force the solvers to branch out, and also compel those in the minefield to move quickly across multiple squares to the defusal target, following a probe kill (which of course, must be against the wall).