On the Defiant, not the Enterprise. The Defiant was a warship. There was no reason for a counselor to be serving in combat on a warship.
No, this was blatant "replacing Jadzia with Ezri", not merely bringing in a new character.
Agreed!
And Ezri was manning communications.
Counselor and communications are two different roles. It was so she could hang on the bridge like Jadzia.