Fun use of those concepts, and the design of the dominion is super cool.
Don't know why... but this made me remember how the Astral Sea works in DnD.
It isn't a 1-to-1, the Astral Sea has notable differences, but the... "feel" when I read it makes me compare, I think it is because of the images.
And... Tony just hinted(or basically threw in our faces), that there are other dimensions in this world, but it feels like it should be... new possibilities that are there, that can have effects on the now but won't necessarily interfere too.
But this too may bring an answer for what the Mesnie is in the great scheme of things, I noticed that despite Fantasia permeating the Sidh... the Sidh itself doesn't seem to be made of it, which means that Fantasia itself is only one aspect of how magic works, which can explain too... how the thaumaturges have their powers, just different interpretations or tapping different portions of the same vast well, that is made of a multitude of "things".
Seth didn't even think before answering.