You cannot make yourself into a certain decision algorithm
What, is this some sort of objection where you believe that determinism means we don't make 'real' choices'?
You could be convinced by my words and make yourself into a person who chooses to one-box. Or you could refuse to be convinced and remain a person who chooses to two-boxes.
Granted, by being "convinced" or "not convinced" it means that you're already the decision algorithm that would make that choice. So what? Whether you'll be convinced or not still affects your decision algorithm from then on.
No, I don't believe that determinism means we don't make real choices. But it is also true, as you note yourself, that if I am convinced by your words, then I was already the kind of person who would be convinced, and I did not make myself into that sort of person. And likewise for the opposite case.
But I am consistent: I believe we make real choices even if Omega predicts our actions, and I also believe we make real choices even if a lesion causes them. The people arguing against my position are saying we don't make real choices in the second case, so they are the ones raising the determinism objection.
You're given the option to torture everyone in the universe, or inflict a dust speck on everyone in the universe. Either you are the only one in the universe, or there are 3^^^3 perfect copies of you (far enough apart that you will never meet.) In the latter case, all copies of you are chosen, and all make the same choice. (Edit: if they choose specks, each person gets one dust speck. This was not meant to be ambiguous.)
As it happens, a perfect and truthful predictor has declared that you will choose torture iff you are alone.
What do you do?
How does your answer change if the predictor made the copies of you conditional on their prediction?
How does your answer change if, in addition to that, you're told you are the original?