Better to say that your state of knowledge about the coin, prior to Omega appearing, is that it has a probability 1/2 of being heads and 1/2 of being tails. The MWI clause is supposed to make the problem harder by preventing you from assigning utility (once Omega appears) to your 'other selves' in other Everett branches. The problem is then just: "how, knowing that Omega might appear, but not knowing what the coin flip will be, can I maximise my utility?" If Omega appears in front of you right now then that's a different question.