Luffy certainly would need most if not all of his allies to bring down an emperor and obviously Law (and Kid, Zoro and the rest) would use their versatile abilities to handicap Kaido (from Law's devil fruit to Enma) but it's not impossible for Kaido to fall looking at the context, his plot is linked with Wano and Oden's flashback, you can never open the borders without Kaido and Orochi going down. Kaido legit has no storyline to be told after liberating the country unlike Big Mom who still have her birthday plot twist to reveal in Elbaf.
Sure marines were worried about the reunion of the previous Rocks pirates, but there's 0% doubt that as you said, a new stakeholder will arrive in wano to face/weaken the Rocks alliance because there's absolutely no way the current alliance can take on two emperors with their subordinates on the same time. Teach, the marines themselves, Elbaf's army etc... these main players could very well tip the scales back.
Internal destruction is stated to be a level above emission. it's basically the ability to manipulate the flow of Haki and direct it to a certain way, if Luffy can fully utilize the stronger stage of armament Haki, i have no reason to think he didn't master the first stage, he completely shattered a massive bulk of steel with a single move and was willing to learn both types:
Also, i don't think being rookies contributes to their individual strength. Luffy effectively took down 3 warlords and two notorious Yonko commanders, one of them who was Big Mom's strongest commander even before training at Udon, has the highest known bounty after the Yonko and currently has a warlord recruited in his crew as well. Kid was targeting Shanks and wandered into Big Mom's territory and his own right hand is quite powerful. while Luffy is currently undergoing the low-top tier stage, Law, Kid, Zoro and Sanji are each going to reach new heights in this arc. it's the saga where the new generation should surpass the old generation imo. i would say it will be ironic for the bunch of rookies Kaido always underrated to take him down (of course by ganging up on him).
I agree that a third party will intervene, whether a Yonko/Marine/Elbaf but Luffy would definitely deliver the decisive blow whether it kills Kaido or not. Oden's death and prophecy were too iconic for anyone besides the protagonist to avenge/fullfil.