Gedol, most reshoim, even those who do lots of aveiros, do eventually have olam haba – the most gehinom lasts for is one year, though every second feels like ages, chazal say. People who lose their olam haba are a distinct category, as delineated in the mishnayos in Sanhedrin. People who are punished forever are also limited to rare cases, “geninnom finishes but they don’t”
Some are punished with worse suffering than gehinnom, which is chiefly intended to cleanse a person of their sins – acher had not been able to enter geninnom, as the OP correctly stated, since his Torah was protecting him. Rebbe Meir davened for that protection to subside somehow and for him to enter geninnom, where he could eventually reach gan eden; this was a very merciful decision, and it was probably a neis, since Hashem changed the way the laws of the ruchnius world run, similar to changing the way the laws of nature work in the physical world.