Omega makes a decision to put the money in the box, or not. In my model of (MWI) reality, that results in a branching - there are now 2 worlds (one with money, one without). The only problem is, I don't know which world I am in. Next, I decide whether to one-box or to two-box. In my model, that results in 4 possible worlds now. Or more precisely, someone who knows neither my decision nor Omega's would count 4 worlds.

Incorrect. Omega's decision is no more indeterministic than the output of a calculation. Asking (say) me "Does two plus two equal three?" does not create two worlds, one in which I say "yes" and one in which I say "no" - overwhelmingly I will tell you "no".