I think that closing an island is necessary, or at least shrinking the existing ones. Invasion is a great way to do that.
If we had three simultaneous invasions, that would give all three islands a chance to fight and defeat the invaders.
(number 3 is an example; it could be 2, could be 4 islands, and I'm not discussing which islands).
The island that can't fend off the hordes, gets overrun; the other islands, which presumably have a higher character density, will survive, although possibly with loss of outlying regions.
The invasions should start on the 'ends' of the islands, so that region loss decreases travel times.
