Thank you so much - I'm glad you didn't mind the game, and it's great to see that the events of the ending caught some interest.
I'll try to answer some of your ending-related questions, just pushing it down a bit because of spoilers:
1. The presence of the monolith itself was what caused the divergence in events - Telwas couldn't have summoned it without Iustirho being there at that point. It being there in the Downstream ending caused the osphor to be drained from the world of the Upstream ending, resulting in Zasaroja's transmigration plan.
2. The monolith not only is not a chimera, but it wasn't created during the events of the game - it existed long, long before; and it was just summoned to Ledra. It's the artifact all osphor stemmed from, essentially its blood; which when wounded in past millennia infected the setting.
Telwas only knew a tiny bit about what has been written about it, as the only proven aspect about it was the ability to draw osphor back into itself. The place seen in the Side B ending is not where it originated, but it was where it had been residing for the past couple of thousand years - the same place Stelia is now.
3A. Nilkaille isn't that complicated - he's a wealthy and connected businessman who over the past decades has cultivated a substantial interest in occultism, and was part of getting Zasaroja in the right place to try setting off the events of the game. The reason why he kills himself in the Downstream ending is because of the monolith tying the chimeras to Ledra. He planned on having the Greenhouse perform the same procedure on him as Patricia did to Yaldukra (which had become more urgent because of his critically failing health), but it was out of reach - the inoculations required to get into the city may themselves have killed him at that stage.
In the Upstream ending, he successfully convinces the Greenhouse to perform the procedure - the man Zasaroja talked to her about in the earlier black and white segment was him. Knowing about the eventual fate of osphor in that plane, he tested the transmigration procedure a few times to jump to the world of the Downstream ending, resulting in his appearance in front of Orris and Viqevi. This version of Nilkaille also is taken along with the other chimeras and their associates when they leave with the monolith.
3B. Stelia is a little more complicated. There's a lot of backstory that could be explained in full, but to keep this from going too long (and not getting into too much of what might constitute a future project), there are a couple of things that can be confirmed.
As the name suggests, she's Greek.
She's in a senior directorial position in the city seen in the Side B ending, where the presence of the monoliths is a known quantity.
The 'Accountant' she refers to isn't a person, as such.
The contact with Zasaroja was entirely unexpected - she has some awareness of other universes, but she has far from all the answers, and was basically put in
this situation and left to figure things out through trial and error for millennia.
As can be inferred from that, she is effectively immortal - but it's the same for the entire society she inhabits. She's seen looking somewhere in her 30s, but she doesn't stay fixed to that appearance - she ages and dies at a normal rate, then her body is reborn from the ocean at her 'baseline' age with seamless continuity of consciousness - for her, mid-20s, for others it can skew older.
I hope that answered some of your questions - if there's anything else you'd like to be clarified, please don't hesitate to ask.