There are more that are unused, that's why the game runs so slow, because someone forgot to take them out in the optimization phase of development, but no, the two islands are the only two the player can get to. This game has been finished since 2019, the only thing keeping this thread active is Better Repack getting updated with new versions of mods.
There is and always has been a second island you can get to using the boat in the ruins area of the first island with the sole purpose of far less restricted building.
And two of them are deadpan types, I have a pair of twins with one each.
Nope
Does it continue extracting if you close the error? If yes, it likely is not corrupt and is just your extractor whining because of the large file size. If so, yeah, it is corrupt.
Those are likely from a mod set and those are intentional if so.