ZeesKite, that’s not exactly accurate. Only a small minority of German Jews were Orthodox in the 19th century, because Reform was so successful there. The Haskalah movement started convincing many Eastern European Jews to drop observance in the 19th century too.
Anyway, as I mentioned above, covering one’s hair was not widely observed in Eastern Europe even among the Orthodox (See R’ Broyde’s article). So were all the gerim during that time invalid, because they didn’t cover their hair afterward? I hope not, because many of today’s yidden are descended from them!
We don’t know how many people converted in Eastern Europe during those many generations in which women didn’t cover, but some did, and we can be sure some were female and had descendants who survived and had children. There are even some accounts of entire villages converting in the 19th century (in Lithuania and the Ukraine, for example).
Iced, I agree, but are there any examples before the late 20th century of people making the determination that a conversion had never been valid because someone noticed the ger not observing a particular mitzvah afterward? Even before the early 20th century? You’d be hard-pressed to find any examples. It’s a recent innovation. As is the concept of gerus l’chumra, as far as I can tell.
This is one of those areas where people pile chumras upon chumras and forget what the original halacha is. If I understand correctly, there are even people now who will claim that if one ger converted by a rabbi goes off the derech, then all the gerim ever converted by the rabbi are considered invalid converts! Madness.