I think the only two that has shown some kind of Advanced CoC is Dragon and Roger imo. Especially how the weather changes and I doubt it's some kind of DF power as Roger who does't have a df but he can change the weather
I still consider this one of the greatest COC Haki feat.
Jozu moreover explains that a man without determination cannot stand in front of this man. maybe this indicates that shanks wasn't doing this willingly.
No. If he didn't display it until now that means he does not have it.
And Rayleigh only has CoC cause it was necessary so he can teach to Luffy about it. He doesn't have qualities of a "conqueror". The same with Zoro.
If someone else had to unleash it amongst the SHs that would most likely have been Zoro for a multimyriad of reasons, such as his power, his determination and hard work, his goals. Zoro is a real supreme king, in his way.