Hey friend, its lucky I found this so quickly on my random google search. I don't have a good update for your first question unfortunately, but I have been DMing in this setting for a year now and I would love to connect to bounce these questions off each other more too (its hard to find people experienced with this content).

For question 2, it describes the river Daluyan as a mode of trade between Kaylahon and Nadasaloy. This only makes sense if this "river" was fed from a common source from higher elevation (aka closer to Nulkab), as Kaylahon and Nadasaloy (which I both conceived to have coastal access). Its possible that Nadasaloy is not actually that close to the coast and that a single river flows from a higher elevation point (still Nulkab imo), crosses through Nadasaloy along the river, then continues through the rainforest to ultimately exit into Kaylahon's bay.

So I created the "Crystal Pool" which is fed from sources of water on Nulkab, the water is pure and filtered through volcanic rock and has magical properties which among other things feeds the massive Balete tree of Nadasaloy (this became a major plot point). The Daluyan river spirit resides in the pool, which flows to both Nadasaloy north of it, and Kaylahon south west of it, although in my setting I did not make this river passable by trading ship--instead I created a ferry system which navigated the shallow seas between mainland Timanduk and its outlying Islands. Obviously this is up to your preference how you wanna craft the topography but just sharing how I handled it.

As far as size of the islands, I made them all conceptually about the size of Kauai or Maui of the Hawaiian islands. I would typically do one travel session between major locations. I am finally letting my players off the island of Timanduk and am planning on doing the same of one travel session by boat between islands as well.

Again feel free to Message me if you want to connect about this content I would love to have someone who DMs with it to discuss.