Jack is a fishman with a devil fruit, so he's similar to Vander Decken in the part where they can't swim. They can still breathe since they're fishman but can't move in the water
Law understood the power difference between Doflamingo and Kaido. He used that and his understanding of Kaido's attitude to try to force Doflamingo to resign. If you go back to episode 623, you see Doflamingo's is scared of the prospect of Kaido being angry (after learning that SMILEs could not be made anymore) As we see here. (I think Law also also planned to actually have Kaido go to kill Doflamingo, but I can't remember where that was said.)
@16:19 "How Yonko compare to Warlords?" They don't. Yonko are at least two if not even three entire tiers above Warlords. There is a CHASM between the power level of a Yonko and even his/her own First Commander. Joker aka Doffy was the Fourth Commander of Kaido. Mihawk is, of course, an exception. Just like a Yonko will be too. Kaido was so frighteningly strong that I just cannot know how we can move on to stronger antagonists from here!
