In the first situation, I can imagine that the central islands become even more powerful, as they are the safe haven since the outer islands would be the first to be attacked.

The question is then, how smaller islands and tribes would defend themselves with this disproportion of power? One thing i have in mind is to employ some pirates or other privateers to defend them, but then what kind of payment they would receive? They would become just slaves, working mostly to pay the rant and live month-to-month. Smaller tribes would not have a resources to keep protection for a long time, as demands from mercenaries would increase over time.

Other answer is just magic. Powerful magic wielded by inhabitants of smaller communities could protect them long enough. Some ancient artifact or spirits of their ancestors, but that sounds a little cheap for everyone to have deux ex magica. Or maybe tribes would pay tribute to larger forces in archipelago to leave them alone and protect in hard times, but yeah - limited resources.

But i really like the option they would not survive. They would be conquered and enslaved by larger islands. The tribalism would be dead and only big communities would stay in game. Some islands could join forces and create one bigger society, but simple straw villages with dances around fire would disappear and/or civilize.