Reading this thread after Book 6 is like watching a vintage movie. I am very entertained 
I came here to confirm if a multi verse is canon in this series.
The guy who documented the crown of madness escaped to another world in this chapter. That wasn’t an illusion or deception by the Aberrant, right? He actually transmigrated?
buildings of glass that reach toward the clouds
That sounds …modern
Also reading book 6 and loving the baby S, she def takes after her grandfather with lessons from her father sprinkled in to fill in parts she doesn’t understand (incorrectly lol) 
Myrdin is hinted to be from another world too, so I’m guessing multiverse.
The Red Guard have that aberrant now presumably so I wonder how much contact they’ve had with other worlds (if they’re real).
I’ve just gone back and reread the theories. I feel somewhat vindicated that Siobhan didn’t end up being a spirit 
RE: the multiverse thing and the glass skyscrapers sounding modern - it could go both ways. It could be a modern setting with skyscrapers as we know them, or it could be more fantastical with giant, delicate glass towers created from pure magic reaching into the sky like forests of underwater kelp. That is my preferred theory that neither confirms nor denies the multiverse theory.
Similarly, the “hint” that Myrddin might be from a different universe is down to him saying something along the lines of him not being originally from this place, which could mean country or it could mean universe. He also had a great deal of knowledge about complex physics which people felt was incongruous with the time and setting. He could be from another world, or he could be from a more advanced place within this world - possibly even that place where there were all those glass spires.
What we do know is firmly in the realms of canon is that there are multiple planes. The plane of radiance, the plane of earth, of water etc. Additionally, it is said that the fey had their own plane or dimension separate to that of the mundane plane which humans reside on. If there’s one, why not more?
Certainly our understanding of our own, singular, universe is constantly evolving the more we discover and observe. Why not this universe, too? More planes become discovered or warp points to elsewhere on their own planet - there are many possibilities.
I leave you with the sea kelp, which is my personal preferred version of the glass buildings - imagining them reaching up to the sky like this and warping the light around them until it almost feels like you’re underwater, even when you’re walking on the ground.
