so the 2 main girls are the last victims of this gin'yo haru coundown sex spree, PC-98 girls are safe.
Apparently not. Did a little digging, and what do you know, nhentai had a copy. According to its version, the PC-98 girls are not safe, at least not on the main handout. (However, they don't have the dates superimposed on the pages, unlike here, so take of that what you want.) It may be a different one than this alltogether, although it had the same looking artstyle, and was also missing the latter half of HSiFS.