The evil Shanks theory is nice but it will never be true. As I see it Shanks is some kind if good pirate, like WB and unlike Kaido, also he wants to preserve the balance of the world aka avoid war aka spare the lives that can be spared and if Kaido showed up in his place at Marineford the war would have went on, actually restarted, with a way bigger casuality count.
About how he stopped him imo he did it using coc but that will be revealed later after Wano when Kaido will be beaten and Shanks and BB need some proper hype as EoS material.