That's not what theists (or many theists) actually say about omnipotence. Also the idea that God could just decide to make things moral or immoral on a whim is highly questionable. Actually, that's the kind of thing that atheists would often object to, (can morality just be the whims of a deity?), and at least some theists would agree with that objection.
The OP poster was reasonable to question if God could forgive all sins. Your approach of "God can just do anything he likes" isn't really going to be a satisfying answer, because it doesn't deal with the moral dimension of it.
But it does deal with the moral dimension of it. According to Christian theology, God is the source of morality. Things are right and wrong because God says they're right and wrong, not because of an objective analysis of the situations. They are wholly based on his opinion.
If God changes his mind and says "I now feel that it is immoral to eternally torture people for the limited actions they take over the course of the few decades of their lives and the moral solution is now to have them exist eternally in Paradise where the better environment will allow them the opportunity to become better people, so the sins they commit during their mortal lives are no longer a relevant factor when considering admission to Heaven and everyone gets in", then that's it. There's no process of appeal for the current residents to use to stop the new neighbours from moving in. God made a decision about what counts as moral and immoral and that decision is final (pending his changing his mind again, of course).
If people don't like it, then it's just kind of tough shit for them, the same as how it's tough shit about the whole "Thou shalt not steal" being immoral when a guy found a new bike sitting unchained outside the mall and he really wants to take it for his own. Really wanting something to be moral doesn't matter if God decides to disagree with you about what is or is not moral.
