Hiruzen doesn't want massacre
Danzo doesn't give a flying f**k so long as the village is all g
Danzo tries to manipulate Itachi saying err "Hiruzen will just kill ya's anyway"
I don't think Itachi would succumb to a cheap trick like that.
It's not really a cheap trick as the ideology is if anyone hurts the village- you kill them. Danzo was stating a fact.
What Danzo was stating was what would happen
when the coup would take effect. They'd kill anyone with an Uchiha symbol and people would fight back. No one would sit there and let things burn. Hiruzen would not sit there as clan members would come at him with killing intent.
I think Hiruzen should've made Itachi head of Root to gain the Uchiha's full trust, he carried the will of fire but also cared deeply for his family who happened to be the head of the clan. In the end it would've been a win-win situation.
It depends at what point.
By the time Itachi was 10, the clan had already sent Shisui as a spy on him. The clan didn't trust Itachi. Which is heavily ironic, given Itachi doesn't betray them until he's 13 (while the one they trust, Shisui, had been giving intel to Hiruzen for years)- at a point where he does not see the clan budging with their plans he decides to tell Hiruzen to see if he will take
immediate action as Hokage regarding it. Hiruzen doesn't, he wants his comrade's approval first. lol.
Shisui also committed suicide when Itachi was 11. The Uchiha thought Itachi was a murderer when he really wasn't for several years before the massacre.
I don't think Hiruzen or the others would be willing to let anyone besides Itachi become leader of Root (he was already leader of Anbu anyway by Hiruzen's approval... the Uchiha still didn't trust the Hokage). They probably didn't trust Hiruzen despite promoting Itachi to the Anbu because of the inaccurate aforementioned speculations (he's a murderer, a traitor, etc) of the latter.
And it wouldn't be possible to promote Itachi as leader of anything before the Uchiha stopped trusting him... he'd be too little at age 9 or younger xD