You poor poor thing, how wearing. I've had days when I've found it hard to like my DS1, who's v bright, v demanding and INCREDIBLY BLOODY argumentative. But the mimicking must be especially hard to put up with - it would infuriate me.
Re the scene tonight, that has to stop. She's being abusive towards you, and needs to learn a bit of respect (sorry if I'm sounding harsh, just how I'm seeing it.)
I would say confiscate toys etc, but you're already doing that, so need to up the anti.
Any nice things happening soon? Parties/days out? She loses one of them for hurting you. Bugger whether it inconveniences anyone else, you're at your wits end and should be supported. If you as a family are going out for a day, could she go to a grandparent/relative with the understanding that she is NOT to be given a nice day/treats?
Whatever you can find to use as a BIG sanction, do it. ASAP. My nephew hits his mother (he's 9 ) and gets away with it all the time. He has no respect for her, and talks down to her all the time. Sanctions never taken.
Your lovely DS will continue being vile to you unless she gets a big shock - make it public for more effect - ie, if she's missing a party, she could phone/visit the friend and explain why she's missing it.
Wish I had the answers for my 2! Have confiscated DS football shirt AND the computer for 2 days, and he's still arsey!