please explain how its a chronologically impossible. The 4th was kakashi teacher, kakashi and yamato are about the same age if not the same age, yamato was a sucessful experiment of Oro with the DNA of the 1st Hokage, during the time of the attack Minato was the Hokage, when Minato was kakashi teacher, we had the 3rd Hokage, danzo fought the 3rd for the position of Hokage and lost.
i said all this to show that what i belief's may be wrong but it does fit the time-line and is possible. IN order for Danzo to be able to live with the 1st DNA inside him Oro's research had to be complete because yamato lived, once yamato was completed IT IS POSSIBLE FOR ORO TO HAVE DONE THE EXPERIMENT ON DANZO ONCE HE KNOW HE WOULD LIVE.
it also can be explained like this, First Danzo got the power of the 1st but found it was not all he needed, he learned of the true power of the Sharingan, wanted the power for himself so he set up the clan(not framed its more like exciting a riot) for he could take the Shanrangan to complete his power.
the time-line adds up,
