If your goal is to show that Omega is "impossible" or "inconsistent", then having Omega adopt the strategy "leave both boxes empty for people who try to predict me / do any other funny stuff" is a perfectly legitimate counterargument. It shows that Omega is in fact consistent if he adopts such strategy. You have no right to just ignore that counterargument.

Indeed, Omega requires a strategy for when he finds that you are too hard to predict. The only reason such a strategy is not provided beforehand in the default problem description is because we are not (in the context of developing decision theory) talking about situations where you are powerful enough to predict Omega, so such a specification would be redundant. The assumption, for the purpose of illuminating problems with classical decision theory, is that Omega has vastly more computational resources than you do, so that the difficult decision tree that presents the problem will obtain.

By the way, it is extremely normal for there to be strategies you are "incapable of executing". For example, I am currently unable to execute the strategy "predict what you will say next, and counter it first", because I can't predict you. Computation is a resource like any other.