Leo shows an eager reaction when Sugar picks the poisoned grape out of her grape bowl. All the grapes look identical so from the spot where he and Robin are hiding it would be impossible to see which grape was the spiked one.
Spandam holding control of Buster call is the catalyst for the entire Water 7 arc. Only the three admirals can summon buster call. Why Aokiji (who is not corrupt) would give this power to Spandam is never explained.
CP9 is an elite team of assassins who specialize in going in cognito and can kill whoever they wish. Why they would need to blackmail Robin to help them eliminate Iceberg, or even to require the straw hats to be framed is illogical and a time waste.