It's quite likely that the islands that fall naturally within the EoC's sphere of influence will vote to join the EoC eventually, since Cayleb and Sharley aren't normally conquering types. Corisande was the sole exception, but Prince Hektor didn't really give Cayleb a choice. The leaders of the other islands eventually joined out of self preservation, offers of marriage to tie their countries together, or both. (Zebediah was the sole exception, but it had already been conquered by Corisande.)
As has others have noted, large portions of the sparsely populated islands are probably unconsecrated, unterraformed ground. They can't grow Terran food crops, and humans can't digest native plants. (Although the native animals have no problem taking a bite out of humans and their herd animals.) I wonder if a slash lizard gets a tummy ache or dies if he eats someone, depending on his size? Oddly enough humans can digest quite a lot of the ocean's fish, although I'd bet those fish are based on Terran prototypes that Shan-Wei developed. I doubt you could eat a kraken without being poisoned!
Silverlode island is largely unterraformed, with a very small population, and it belongs entirely to Cayleb. (And aren't we glad it does!)
The advantages of duty-free trade with the islands of the Empire, along with the soft hand of the CoC may be all that's required for those islands to join the fold eventually. The Trellheim pirates may be forced into the fold if they refuse to behave themselves.
Siddarmark grew in much the same way, as most of its central and western provinces were enclaves and countries who voted to join the Republic. The southernmost regions of Siddarmark were formerly Desnairan, and quite likely were more than happy to see their imperial betters slashed to ribbons by the vaunted Siddarmarkan pikes!
It's unfortunate that the CoGA stepped in and established Silkiah as a buffer zone, since if Siddarmark had controlled the Salthar canal, the ICN would be sending ironclads through it already...