Acher was not a simple rasha; he had led a life that warranted immense olam haba for his Torah and avodah prior to going off the derech. Rebbe Meir davened that he should get it.
He had a tikun, but where did you see that his gan eden is better than tzadikim? I don’t think that’s true; chotei niscar? Doesn’t make any sense.
And what that has to do with feeling dveikus in gehinnom
…it’s not a nechoma – don’t do aveiros, do teshuva, and you don’t have to worry about gehinnom. Hashem doesn’t want anyone to have to go there, but it’s worse than anything we can imagine. Such is necessary to cleanse a person of their chataim. I don’t think people have the capacity to experience that kind of suffering and feel Hashem’s presence. It sounds like a fantasy.