Re: Kotoamatsukami is a part of Itachi power.
Kotoamatsukami is unique to Shisui's Mangekyo.
It is his jutsu.
However, with Shisui's eye under Itachi's control, it does become part of itachi's useable Jutsu, but it's not his.
Just as the Sharigan is not kakashi's, it's still part of his arsenal.
Possession of a jutsu does not make it yours.
To put it simply:
Just because you can use it doesn't make it yours.
I can use the textbooks at school but they're not mine, yet they help me. Just like the Sharingan helps Kakashi, and Kotomatsukami helped Itachi.
There's some logic for the intellectuals.