I'll be interested to see what you think; whether their purpose in the structure works out the way you suspect.

Tigana, Arbonne and Sarantine were all firmly land-locked in their fantasy worlds, so there wasn't any divide between that and our world to be bridged.

I think, when I started reading The Summer Tree, that I subconsciously expected the divide to feel like Narnia. But ST's our-world environment is much closer to our contemporary one than world-war England. In a way, both the English and the Narnian environments were fantasy to me. Maybe that contributed to smoothing the awkward edge between them.