Minato could prevent the Uchiha massacre

[Baka Sennin]

because Minato's thought that it was Madara was only his first assumption, after that he would have surmised that it was a normal uchiha i suppose. after all, mikoto being the uchiha chief's wife it is much more plausible that they know about kushina birth that a dead exile of the past

 

[Wesobi]

Wouldn't it just have quickened the uchiha's "coup d'étât"?

Minato tells Hiruzen > They go to confront the Uchiha since they have no idea that it's not an active member (even Minato did not believe Madara was alive, despite his conclusion) > Uchiha get all defensive > Others think they deny it > aaaaaand big old civil war all over. This would probably cause just as many uchiha to die as others.

 

[thegame]

Wouldn't it just have quickened the uchiha's "coup d'étât"?

Minato tells Hiruzen > They go to confront the Uchiha since they have no idea that it's not an active member (even Minato did not believe Madara was alive, despite his conclusion) > Uchiha get all defensive > Others think they deny it > aaaaaand big old civil war all over. This would probably cause just as many uchiha to die as others.

Well if he added, "He has MS, and uses S/T ninjutsu", it could narrow down the research. Undoubtedly he must have seen Obitos MS. Only reason it wasn't shown in the manga, was to hide the fact that it was Obito, who did it.