And I wouldn't be surprised if there is a backup plan to replace the Shichi because if they aren't being told to move or aren't going to be told eventually they stand a pretty good chance of being caught in Sengoku's plan since they are in close proximity to WB's forces, hell even the fodder marines may atleast be ordered to move in a position to not be caught in the trap but would anyone warn the Shichibukai?
They'd be smart enough to realize themselves.
Unless Vegapunk has applied Yellow Flash jutsu to the pacifista and they're gonna take out the pirates in one second it's not gonna catch them off guard completely.
Keep in mind this plan involves killing Ace ahead of time. For all we know a bunch of Kumas can pop out of nowhere on the battlefield, two ready to blast Ace's head off.