okay lets say they do have their coup
you think lets say a group of 100 uchiha can handle the entire village every clan every ninja and itachi and shisui?
keep in mind shisui would still be alive, if this was the way it was going to go down
there wouldn't be a surprise attack because itachi is the mole, they'd wipe them out before they get a chance
it'd be an over night or maybe two night rebellion, and the next day it would be like nothing ever happened maybe like 50 dead
and if shisui got his hands on fugaku he'd just KA him and make him retreat
The Uchiha weren't trying to take on every village. Just overtake Konoha.
In reality they probably wouldn't have succeeded, but that's not the point. Just the fact that they would have tried would devastate Konoha from the inside out. Would Konoha be destroyed? No. But it would have been weakened, people would have died, and parts of the village would have been destroyed.
This weakening is the only opening the other villages would need to attack them. That is how a war would have started
Even if the attack only lasted a night or two it'd still be major destruction. The Uchiha are a clan of geniuses. Not all of them are necessarily on Itachi's level but they are stronger than your average shinobi. For a plan as bold as this they'd think everything through. Plan several steps ahead and make the attack as efficient as possible.
The fact that the elders saw no other way than to assassinate them could have been because...well...there was no other way.
Also, the thing about clans or groups like these is that the entire clan is in on it (obviously not the kids and old people but you get my point). Even if Shisui used Koto Amatsukami on Fugaku and changed his mind another Uchiha would step up and lead the assault. The
entire clan wouldn't have been swayed just because the leader changes his mind