Hi, I wanted to make a suggestion. Well, I don't know how you're planning to do this last update or if you'll have time to think about what I'm going to say. I think very few people here want to see the complete story, so I'm going to suggest this.
1- Rumia could be working on some kind of pheromone, which she starts making after some event in the game. After she finishes making it, she finds Leah and asks her to test it, and she does. There are two possibilities of what could happen: the first is a gangbang of insects (I don't have any specific ones), or this pheromone attracts a giant monster, which takes Leah to a place and does those things to her. However, this ends up unlocking a new place on the map for the main story, or after the monster does those things to her, it falls and ends up unlocking a new passage (One of the two options). (I also have no idea which monster it would be). Thus allowing the main story of the game to advance.
2- During the pheromone creation phase, Rumia accidentally drops a vial in her warehouse without noticing. Insects, or even a spider (as I've seen people mention), or that insect she has downstairs, end up becoming a "mutant" or a larger creature, eventually leading to those scenes with Rumia or Leah. It could also function as a growth tonic for some creature that was down there.
Those were the two ideas I had. I think it's kind of difficult to do all of that, but there you have it, they almost wanted to use it as inspiration for something.
-Perhaps the translation left some words sounding "heavy," but that wasn't my intention.-