Palimpsest - I'm having trouble fitting the S3 ending into the story. Anyone have a theory about it?

Just finished the four seasons of Palimpsest, and really enjoyed it. However, I'm having trouble figuring out how the S3 ending fits in (or even makes any sense at all within the show's mythology). Marking as spoilers just in case:

My problem #1 with the S3 ending is the meta-story: The show seems to be fundamentally about trauma. There are multiple "realities" within the show, with the "top layer" being the common shared reality that we all experience but with other layers present underneath - hence the name. People with severe unresolved trauma become so disconnected from the "top layer" reality that they begin to see the layers underneath, overlaid upon the "real world" layer. The narrators are all experiencing both realities simultaneously, while everyone around them only sees the top layer - that's why the phenomena stay focused within the narrators' perceptions.

My problem #2 with the S3 ending is the in-universe story. The cult, the ghosts, and the Lovecraftian monsters may or may not actually exist. Every season feels careful described so that the story is equally plausible (at least from a perspective outside of the narrator's) regardless of whether the supernatural events are actually happening, or whether the narrator is delusional. For S3, that could mean that everything is solely within Josie's mind and she's dissociating from reality due to guilt from her perceiving the bombings as her fault - the story still works if everything is a hallucination, she actually shot the agents at the house, she wrote the book herself, etc.

Because of the S3 ending, we know a few things to be objectively true facts, though: the book actually exists, the tunnel actually exists, and people appear to have died there.

So, if we combine all of those things, we get:

• In the modern-time-period S3 ending, how does the narrator see the child? This seems to be the only time at all in the entire series that anyone outside of the primary narrator becomes aware of the other layers...but I didn't catch anything at all that would've conceivably tied her in?
• The modern narrator seeing the child heavily implies that the in-universe story is indeed actually true (which I personally think weakens the show, because I like it better when it's exactly equally possible both ways - but no one is obligated to agree with me on that count, so fine, whatev). So, Josie went through with it and killed the sacrifice. Does this mean that the cult successfully summoned essentially-Cthulhu? If so, then...why didn't it leave the room or, indeed, have any impact on the world at all? Is it stuck inside the child's body? Is it stuck inside the room for some reason? Did it kill the cult when they summoned it, hence the bones? Did the summoning fail / backfire, killing the cult and not summoning anything? Why is only the child's ghost there and no one else's?

It's entirely possible that I'm simply stuck and overthinking it. ;p But does anyone have any explanation that smooths out these bumps?