I was around a bit earlier. Somehow I picked up on the C17 website quite fast. As far as I know, Videorama has always been a dvd/vhs maker, not so much a content reseller. I think sites like C17 had connections with mostly Czech and Russian agencies who provided models/content, and then used dvd/vhs makers like Videorama, Staviss, etc. to make de dvd's.
If I check the videorama.de site on archive.org, then around 2000 I see they mostly sell their own dvd/vhs. No content. So you would not find Kristina/Yvette there, unless she is on another dvd listed there. That is kinda how I remember it aswel.
However, you do see C17 models pop up on other sites like 18eighteen, karups, etc. But there does not seem to be a clear pattern on how those routes go. What is clear is that C17 was influencial enough to often have their contact agencies abroad to make them wear their clothing line.
I think C17 was one of the first to do that, and that makes them very recognizable.
The only hardcore I could find was with a dog...

Which makes me wonder, as it seems a bigger step to to that, than hardcore with a man. So potentially there could be more.
The TFH mags that you mention are from end 99 and 00. I have noticed that C17 can wait years before releasing sets and videos on their site and dvd's.
Pick the oldest source (which are probably these TFH's) and go through Videorama dvds from that time. Or old "native" Russian and Czech dvd's/vhs. She may pop up there.