The Rabbi mentioned that very few in Jewish history were so evil that they will not have a share in the world to come. I assume these are the people who after 12 months of "purging", still didn’t come up clean. If they will not be in the world to come, where would their unrepentant soul end up? Is hell forever existing?

Firstly, we use the term “improving” or ‘purifying” and not: “purging”. It’s written that afterwards, the souls of those rare individual “super-evil”, either simply cease to exist or in even rarer cases, undergo eternal punishment. It’s difficult for us to know whether this is meant to be taken literally or to serve as a deterrent for others.