Okay, real talk here. A porn game has only limited ways to work and if the MC is a woman, there are only three ways. The problem is that those three options are, for the most parts, quickly overused, if you have many opponents. The options are pretty thin: Impregnation, eggs, parasite, reborn, bomb, one way lane, eat through body, corrupt ovaries, eat or steal ovary eggs... as you can see, it goes stale very fast and if you make a game and on top of it not guro, your methodes are really slim. And aside of Game Over moments, I doubt that Inui will kill Ren in game by the mobs (unlike other games). To me, the impregnation option is nicely done, since the mobs use Ren's eggs and don't push eggs inside, which is a nice refresher unlike in most hentai nowadays.
As for me, I wish we had more lactating scenes. I can't get enough from that, especially when plants, tentacles or animals like those bats are involved. Maybe the rats and the slugs (leeches?) as well... once can dream.
Not really. Those mutants are a product of the Heaven drug, that main villain (forgot his name) created to mess around. Each creature, that got in touch with the drug, mutated into a horny creature, that has only one idea in the head... to breed every female it can find. Those slugdogs are nothing else. Their only purpose is to find a female and impregnate it. Why doesn't matter. This is why every mob in this game is after Rin's pussy.