One thing is for certain: the non-Jewish understanding of the “bad” place, is completely wrong, and has no place in Yiddishkeit. Being raised in Golus, we are exposed to the beliefs of other religions, sometimes without realizing it. Especially because they sometimes use the same words as we do, when we are translating these concepts from Lashon Hakodesh, into whatever language we use. I don’t know what Gehinnom is for blatant Resha-im. I’m referring to Jewish people who were imperfect, and sometimes very, very far from perfect. They may have been completely secular; only Hashem can judge what opportunities they may have had that they missed. According to our Mesorah, Gehinnom is a temporary place, that serves a purpose.
Col Yisrael Yesh Lahem Chelek L’olam Habah.
One CAN lose their chelek, for example by embarrasing someone. But besides for asking mechila, and doing as much Teshuva as possible, the person then starts building up their chelek again.
There are things for which one can not do Teshuva, but very very few. Even things that are VERY bad, Yom Kippur can be mechaper in addition to teshuva; Yessurim can be mechaper in addition to teshuva; Misa can be mechaper in addition to teshuva.