If Yamato had been in the room when Kanjuro showed up, he would have been deceived by the Oden fake too and would just have a sword in the belly for his trouble. Not to mention, if Yamato introduced himself to Kinemon as Oden, after Kanjuro had already done his trick? Might have gotten a sword to the belly for his trouble.
And, though he can hold off Kaidou, we saw on the roof that it doesn't matter if you have backup, the weaker characters get beat down as collateral. Yamato being in the room might have spared Kinemon, it might not have, Maybe he uses a blast attack instead of hitting him with his mace. At best trying to protect a target would just leave Yamato open to direct attacks that he'd then be tanking instead of dodging.
He left Momo in a safe place with help just a moment away. And if he'd been allowed to leave sooner he MIGHT have gotten to the roof on time and started stalling Kaidou sooner, or kept Luffy from being knocked off.
Either way this is One Piece, so Kinemon's either not dead, or will have a heartwarming speech when he does and be okay with it. (A WAR should have at least one casualty after all.) Or a fire festival spirit. Same with all the other "surely dead now" Scabbards.
Pretty much, its pretty disingenuous to say otherwise.
If were gonna follow this logic down the pit where it comes from Luffy is to blame as if he didn't fall Kinemon would be fine, but we can follow this logic with other people to but its just foolish.