Well, Harry already tried to talk to Sirius using his mirror and that didn't work. I also believe that if the person was dead they would have to have the mirror with them to talk to someone on the other side, ya kno.

Maybe, Harry will use the mirror by talking to Hermione and Ron. They may go back to Hogwarts, so then Harry could have them get a mirror and Hermione and Ron could do research or help Harry from Hogwarts someway, ya kno.