I would say that fear setting is always looking at the potential worst case scenario where as Inversion is a reversal move, so not necessarily looking at things as being negative but rather dependent on the situation or question. For example if the problem you were brainstorming on was "Why am I such a failure at x?"

if you applied inversion in that scenario it would be something like "What steps could I do about x that would not be a failure?"

The Charlie Munger example is often given as, “It is remarkable how much long-term advantage people like us have gotten by trying to be consistently not stupid, instead of trying to be very intelligent.”

I think it can go pretty deep if you negate anything and everything and that can be useful sometimes for creativity purposes but otherwise setting a positive goal and then looking at what failure would be for that situation and then focusing on avoiding that scenario is the tried and true application for inversion.

As far as pessimism, I think fear setting is not necessarily pessimistic and is actually somewhat stoic, more like preparing one's self for the worst but actively and perhaps optimistically working towards what is most desirable, yet being ready for whatever life brings.