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.
If theres need for further proof that BW fired all their developers/programmers, this is it.
Turning a bug into a "feature". gg bw