Dear Anssi,
Thanks much for your questions. As for the reverts they are not at all punished for their previous sins before Islam. This is as clear as crystal in Islam (like everything else in fact)

What this muslim meant by all people are going to hell is that theres a verse in Quan says so. And this means that all people will bypass the hell but most of good and kind people will not feel anything while bypassing it.

Bad people will be tortured in it according to their bad deeds. Some of them will bypass slowly and some will even fall into it.

The thing I want to tell you -as I see you are open hearted and minded- is to read more and more about Islam. You will not take much time to discover that it is the true religion of all prophets even Jesus. You will find it all as much of logic as you can expect.

My kind regards