Doffy probably gets stomped by all of them.
The difficulty as I describe between Whitebeard and Kaidou with the rest are comparable.
The Emperors aren't incapable, they just need the four road poneglpyh to reach Roger's "station"
Doflamingo said that Whitebeard just stood in front of the throne and never sat on it.
It's heavily implied if Newgate wanted to - he would've become the PK
You're making things more complicated that it needs to be. That was obviously Oda's way of saying Whitebeard could have had everything Roger did if he wanted to. It was just a way to affirm their equality.
"Don't count the days, make the days count"
Kaidou has shown to not give a fuck about Doflamingo's well-being. He would personally kill him if he screws up his end of the deal in fact.
This coming from Doflamingo himself.