We see in this panel that Sasuke is able to pass through the Yata Mirror when Itachi was walking toward him
Itachi let Sasuke not be harmed by Yata's reflection in that scene because Yata Mirror wasn't out anymore I think.
Only Susano'o was. Itachi let it down because he wanted to give Amateratsu to Sasuke. There wasn't any fighting anymore.
Yata Mirror being defensive against threats only is the same thing as Gaara's automatic sand gourd shield defense mechanism.
Gaara's sand : will it still guard Gaara's body if you are just trying to walk near him or will it still come up?
It is his mother's will so if his mother thinks it will be something dangerous then she will defend , but if she doesn't then she wont. Its pretty simple actually.
Kabuto's DSM sound genjutsu did affect Itachi , but Itachi still fought through it . I don't think Yata was out in that moment.
Yata Mirror and Susano'o are two completely different things. I bet Itachi could probably bring Yata Mirror out without Susano'o since Susano'o is his own chakra and MS jutsu , but where does Yata really stand on the weapon wielding scale?