Okay, great one. I like that you already start with a very important theory: the key related to the mystery around it.
I don't think it is a stretch; as a matter of fact, I thought it was very curious that once the Voices/Outer made Myrddin be sucked into the darkness and severed our connection in the end, we saw the room getting farther and farther, almost like seeing the room as a small cube in the end... I thought it was a very interesting visual choice.
But I don't know if the room or the game file themselves are the key.
Because at some point we will need to deal with the key, right? And since the developer mentioned that it might be possible to see the story expand to more than just talking to a person in a room (see one of ToMyself's answers to my theory comments in the updates section), then it's difficult to think the game file might be the key, but the room could be (the cube game icon also being an interesting note, even though many games have icons like that as well).
So if the key is actually the room (seeing it from our perspective, it does look like a cube), I wonder how we are going to explain that to Myrddin in the next chapter. Since the story might expand, like I mentioned above, we might be able to free Myrddin from the box sooner than we expect, and finally see what is outside, which I theorize to be something different from what we (humans) are used to.
Also, having the message "Do not trust the false Messiah" appear near the moment we see the room/cube distancing itself from us is very interesting. Once again it might just be a visualization of our connection being severed, but it might also be nodding to something else.
Now, the implications of the game being the key would mean the player would have to have access to the game files. Past updates from the developer do mention liking details as such, but I don't think anything extremely big is in the files for now (but I will comment about the MainAct in a future message, don't worry!) simply because ToMyself mentioned having difficulty balancing the browser and the downloadable version of the game. I theorize it's because the downloadable version would have details rewarding those who check the game files, and it is more difficult or unavailable at the moment to check the files if you're playing on the browser.
However, in the most recent update, the developer mentions the story will have more and more limitations in that regard. I wonder if it's because we're getting near some sections in the story where we will have to check the files more frequently, and since it is difficult to do so in the browser, it would create discrepancies between both versions. But that's just a technical theory.
Now, an extra observation here: near the end of the chapter, Myrddin keeps saying stuff similar to "I can't ask that of you" and "It is my burden to carry." Again, my current theory is that Myrddin is not a 100% bad guy. But he was very cautious, and his sentences might imply he actually wanted the help, obviously. However, I cannot deny that there might be a possibility he was expecting us to take pity on him all along.
But if I'm being honest, I don't fully believe in that theory, since Myrddin was very angry and unstable when we read his book, meaning it is very possible that he wouldn't be able to manipulate us right after being very emotionally unstable. Even so, I thought it was worth mentioning here, like how hesitant he was to ask for our help. That moment was interesting, maybe planned.(Early theory here, not much to go on, particularly a path I don't think might be the one, but interesting enough to mention anyway.)