Csar, I’m in full agreement with your first paragraph — thanks for the clarification.
“Failure to keep even one of them (i.e. immorality, petty theft, idolatry, etc.) constitutes them forfeiting an afterlife.”
But what’s the source for this? What about gilgulim, teshuvah, G-d’s mercy, etc.? What’s even the source for the idea that large numbers of people can just “forfeit the afterlife”? I thought extinguishing someone’s eternal existence after death was an extreme punishment.
There are also a few sources in the Gemara and elsewhere that say, if you do X, your sins will be forgiven. For example, Hashem is merciful to those who are merciful to others, and Hashem forgives those who forgives others.
About teshuvah, I don’t have any sources, but it seems obvious that goyim (whether bnei Noach or not) can do teshuvah. Feel bad about the aveira, ask G-d’s forgiveness, and resolve not to do it again — this isn’t just Rambam, such ideas are very common around the world. The Zohar says there’s one sin for which repentence is not possible (spilling seed) but Rebbe Nachman of Breslov said that can’t be taken literally — repentence is always possible. I don’t see why it would be any different for goyim.