If Zoro ends up fighting him then Zoro>> Luffy, which is impossible.
This is a flaw in your theory, it is possible because we have already witnessed it. Zoro was stronger the whole time, nothing new...
Whether Zoro will fight King or not, cant say for sure but I do think Zoro is the only one who can beat him.
All Zoro needs is to get infected with the Ice Oni virus and he is at 100% for an entire hour, long enough to fight King and Kaido.
I dont think Luffy and Zoro will face Kaido together but one after the other.
Zoro fights him and when 1hour virus limit approaches, Zoro has to stop and take the antidote or he will die.
Luffy takes over for his 5 minutes and with his last strength knocks Kaido down once more.
Kaido stands up and Zoro ends up killing him soon after that when he is above the capital trying to burn it down.