Unless the curse of the Abyss is somehow dispelled, the only ending for Riko at the end of the series can get is being trapped in the 7th layer and beyond indefinitely like Lyza did, living the rest of their lives there. And that is the positive one. Every other ending is just death, death and more death.
Bondrewd can see through Nanachi's eyes. That means he’s seen everything they're done since their escape... and... he... is... proud.
The Amaranthine-Deceptor insects are native to the 6th layer, but were mysteriously brought to the 4th layer recently. Given the Umbra Hands state that they shouldn't be here (meaning that it wasn't Bondrewd's fault) and the only other thing we know that could go from lower layers to upper layers is Reg, did he bring them into the 4th layer?
Why is Mitty dressed so skimpily in the flashback? Nanachi didn't change at all on the trip, so they weren't given new clothes. However, despite being from the far north, Mitty is wearing a skirt and a bikini top that heavily accentuates her bosom. Likely as not, she had been forced into prostitution back home just to get enough to eat. No wonder she jumped at the chance to leave, even if it did mean going into the depths of the abyss.
What are the tasteless nutrition bars in Bondrewd's base made of? When transforming children into cartridges, all of their unnecessary body parts are removed and ground up. You do the math.
Why does the entrance to Iruburu Village look like a vagina? Body Horror.
Mitty is somehow perfectly cloned in body and soul, aside from regaining her missing eye, via the Iruburu Balancing, where giving something of value to the village itself manages to produce currency or new items as a result. Over half of Belaf's body was apparently enough value to bring back Mitty. The Abyss doesn't just have a Curse, or Body Horror artifacts and strange mutant wildlife; it can distort the very laws of reality and effectively produce sacrificial magic from the countless dead child souls inhabiting Iruburu that can even bring the dead back at a great cost, while also inferring that the dead of the Abyss stay in the Abyss. And this is just a taste of what's to come.
If any cave raider did manage to reach the deepest point, Reg and Riko would have seen a balloon by now (6th layer at the time of this writing).
Irumyuui's was celebrated as the only girl in her family, only to be cast aside and marked for sacrifice due to her infertility. Her biggest wish is to be fertile when using a Cradle of Greed. With that mentality, her situation before being taken in by the Ganja Corps screams Breeding Slave. Which is one hell of cruel irony when remember what eventually happens to her...
Irumyuui mentions that she was "loved by all her brothers," which sounds innocent on the surface, but considering her family's interest in reproduction to the point of sacrificing someone who is infertile, it's possible that there may have been more thanone sinister thing going on in their family.
Irumyuui mentions that her mother “slept with a lot of people” and “is really important, so she has lots of children” implying that her mother was prized for her fertility and bred by many men across the village as well. Which adds further explanation for her being thrown away for being infertile.
The Cradle of Desire works by making the desires of whoever is touching it come true via Reality Warping. Therefore it can't be anything but a Jerkass Genie. When have you only ever had one thing on your mind at once?
Prior to meeting Riko and Reg, Bondrewd fought Srajo and her entire Hail Hex fleet and lost, meaning that he was recovering from the injuries and losses sustained from fighting an organized fleet of delvers when the heroes fought him. Had the fight with the Hail Hex fleet not happened prior, Riko and Reg would just end up as some more lab rats in his experiments.
Prushka is confirmed to be still alive— and, from her mental state reached in Riko's dream, seems completely sane and able, as if the Curse hadn't really affected her despite being a cartridge turned into a Life-Reverberating Stone and later on a proper White Whistle. Except Prushka was already an incredibly kind and optimistic child who had survived immense pain and trauma once before and was perfect for such a horrible fate; we don't see how other's White Whistles are made yet, but it implies they're all the loved ones or close friends of experienced Cave Raiders, and may be anything but sane compared to Prushka after what they've likely been through. With them being quite a legendary artifact for top tier Raiders, god only knows how many sacrifices the close ones of countless Raiders had to make just to get that much further.
The White Whistles themselves are also a special kind of horrifying, because of the state that these souls have to exist in to help their wielders. While Doni was Lyza's whistle origin, and Prushka became Riko's, Bondrewd managed to remove and clone his own soul and turn his very body into his own White Whistle, and then these souls can have special interactions with their wielders if Riko isn't just unique. They also have to seemingly sacrifice every single possible thing they're willing to ever have again as a "prayer" to the wielder they'll become bonded to, which could be matters of absolute trust — or potentially abused, if Bondrewd praying to himself is anything to go by. Mankind straight up weaponized and compartmentalized their very essence of being to continue adapting and thriving within impossibly dangerous environments that they were never meant to tread, and there's a strong chance some of these sacrifices were not a proper consent.
As Geoff Thew pointed out in his video, the Hammerbeaks, a species of bird common to the Abyss throughout the first few layers, seem like something out of a videogame. Their meat is succulent and provides a plethora of nutrients. Their feathers happen to be ideally flammable to serve as kindling wherever one might be. And their signature beaks, along with their claws, can strike sparks easily as a substitute for flint and steel. Why is this Fridge Horror? Because it seems awfully convenient for a species with such evolutionary adaptations that not only don't benefit it, but work against its survival, to not only flourish inside of the deadly reaches of the Abyss, but thrive in enough numbers for adventurers to make use of them regularly. The question then arises...if a species that exists to ensure, not its own continued existence, but the continued existence of others...how did such a species come to pass? Did some early adventurer create them artificially to help other adventurers? Did they rapidly evolve from something else to form a twisted mutualistic bond with adventurers? Or...did the Abyss create such a species to help those it calls into its depths reach further down by supplementing their own gear with a local source of material?
Irumyuui wished on the Cradle of Desire to cure her infertility, but ended up giving birth to many babies that were stillborn or died shortly after birth. She wished to become fertile, but being able to conceive a child and being able to give birth safely are two different things, and in real life, a young girl's body is sometimes ready for the former long before it's ready for the latter. Some girls start getting their periods around 8 or 9, and Irumyuui might not even have been that old at the time she made her wish...
None of Bondrewd's cartridges are shown to mutate into hollows, instead quickly expiring once 'used'. From Bondrewd's perspective, he is sparing the cartridges from suffering the same fate as Mitty and his other elevator experiments.
