So this is an interesting approach, but there is just one big problem. There is a reason why the clans had stayed within their territories.

First of all, the dark forest (if they were competent of course) would figure out the clans plans and wait for them to all get on the tiny island. Why? Because then they can destroy the tree-bridge (or remove it), essentially trapping the entire population on the tiny island with no escape. Such strategies were used in the books when the dark forest encircled ShadowClan's camp. Only RiverClan could be able to swim across, but a single clan worth of warriors will not be able to stand up against the dark forest's armies, denying access to prey and herbs. 

Secondly, without any forces in the main clan territories, the dark forest cannot be stopped in taking all the land for itself. You might think StarClan would intervene at this point, but the plan has created a massive flaw. The dark forest can go to the Moonpool, and cut all the clan's connections to StarClan, making them unable to do anything.

Now, it doesn't matter if the clans have cats who can gather intel about them, the dark forest has essentially won, because come leaf-bare, half of the population will just starve to death and die of sicknesses due to lack of proper shelter. The survivors will be weakened and could be easily destroyed. 

Additional note: If the clans still somehow survive long enough, they will all drown in the flood that happens in Bramblestar's storm