I don't see why they couldn't just have kuma teleport an entire fleet to the heart of the island instantly on sussys orders
Because Kuma can't teleport shit. Kuma would have to blast away each boat separately inside his paw shapped air bubble. Not sure how a big wooden and metal construct would behave upon landing. Not saying it isn't feasible, but it's far from being as simple as you make it.
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Pre-timeskip I would have agreed to that. However, we have had direct statements that Akainu has been building up the strength of the Marines in the New World and we can expect him to be a more agressive servant of justice than Sangoku was. So I wouldn't put it past him to try something anyway.
Travel times should imply indeed that we won't see anymore party poopers step in before the Straw Hats have escaped, but I wouldn't be surprised if they're only a couple days behind. Which shouldn't give the BMP enough time to fully reorganize their defenses, so while annihilation isn't an option they can certainly be driven away from their homeland.Bonus, it gives them an extra reason to show up later in the story - either for vengeful purposes or because they're trying to set up a new base somewhere else - and tie loose plot threads.
Preparing for war isn't the same as openly going to war and leaving yourself open to counterattacks. Besides, I imagine quite a few kings and other leaders would mind if they were unknowingly pulled into a war. What are they supposed to discuss about at the Reverie if the war has already started before the Reverie itself had began?