[Fate/FGO] Surviving in an R-18 World

I was reincarnated as A, a handyman working at Chaldea. But there's something strange about our Chaldea.

The female staff and Servants were being seduced by a hazy-looking, muscular, bald old man.
The protagonist and Mash, who seemed to have mutual feelings for each other, were torn apart by that man, and Mash became corrupted... This world was insane. While I was living quietly, trying not to stand out in such a world, I found myself back at the prologue.

Since I've become a hero who has returned to the past, I've decided I have to do something about this crappy world.