It took several dozen tries, but I finally managed to clear the game while sparing all rats, the lovers, and the dragon egg. I believe that's the most achievements you can earn in a single playthrough. Though I have to say, it bothers me a lot that after the first time you get a full clear, your ending is permanently corrupted. That gimmick works great for Undertale, since that game does everything in its power to steer you away from killing all the monsters. But it's completely out of place in a game like this, where a full clear is almost certainly the first challenge run any player will try.