Thanks, this one made me think.

a machine inside the box will reprogram your mind such that you will believe it completely

It seems that Omega is making a judgement about making some "minimal" change that leaves you the "same person" afterwards, otherwise he can always just replace anyone with a pleasure (or torture) machine that believes one fact.

If you really believe that directly editing thoughts and memories is equivalent to murder, and Omega respects that, then Omega doesn't have much scope for the black box. If Omega doesn't respect those beliefs about personal identity, then he's either a torturer or a murderer, and the dilemma is less interesting.

But ... least convenient possible world ...

Omega could be more subtle than this. Instead of facts and/or mind reprogrammers in boxes, Omega sets up your future so that you run into (apparent) evidence for the fact that you correctly (or mistakenly) interpret, and you have no way of distinguishing this inserted evidence from the normal flow of your life ... then you're back to the dilemma.