No, it wasn't a mistake.
Itachi was 13 at the time Shisui died. Itachi stated that he looked at Shisui as he would an older brother. Shisui was a jounin with his own team in that Kakashi Anbu arc. Shisui was a young adult, so he could have been 18-20.
He could use KA in either eye, hence why he wanted to use the one on Fugaku, but Danzo stole that one. Then, he had the other one, and he gave that to Itachi but it was on cool-down. Meaning that he used it before on a mission prior.
So, Shisui did use it before and that's how he knew that the cool-down time was ten years. Also keep in mind that an Uchiha doesn't have to use their special MS tech often to know how it works. Obito used kamui right after Rin died, while Itachi used Tsukiyomi during his S-Rank mission.
Itachi was 13 when Shisui died and he became a missing-nin. He also stated that Shisui was older than him. Shisui was anywhere from 18-24 years old.
No, it takes ten years for Shisui. Especially considering that the only way Itachi would have known the cool-down time would be if Shisui told him, if it could be used multiple times then Itachi would have replaced one of his own eye's with Shisui's and used it, or Shisui himself wouldn't have given it to Itachi and instead attempted to KA Fugaku again. It wasn't implied for non-Uchihas. It was stated that it's 10 years for everyone, unless you have Senju DNA which lowers the cool-down time to a few hours...something Shisui did not have.
