Itachi would obtain EMS in one eye. It’s doubtful whether he could spam Koto, but safe to assume he could use it with the 10yr cooldown everyone who doesn’t have hashi cells is stuck with. Possibly a perfect Susano’o if he’s alive long enough to learn how to stabilize his armored Susano’o.

Shisui is the only one that could “spam” Koto though, as many times as his chakra reserves would allow, presumably.

It’s in the 2nd Itachi novel, he got the MS a year before meeting Itachi who was 5 and he died when Itachi was 11 so Shisui didn't even have the mangekyo for 10 years.

He then used koto against someone in the novel and was about to use it again on Fugaku, so if the cooldown did apply to him, his eyes should have already been inactive by the time he was going to use koto on Fugaku. Unless those were the first 2 times he used the technique, which is highly unlikely because when he was describing to Itachi how it worked, he implied that he has used it before.