7.1 Setup in odd shaped room

[Hoser59]

I am planning on upgrading my old 5.1 receiver to one that will accept HDMI and handle HD audio (I have a PS3 as my Blu-Ray player). I am thinking of upgrading to 7.1 but I am having some issues with where to place the rear speakers. I have attached an image which shows the layout of my living room and where I propose placing the rear channels. All the other speakers are currently located where they are shown. The three red dots show the primary seating positions and the green lines are meant to represent the direction the speakers are to face.



The left side and right side speakers are about 6' off the ground and I would place the rears at the same height.

Will this location be of benefit or am I better off sticking with 5.1?

Also...

Is it possible for a 7.1 receiver when playing 5.1 audio source to split the left and right channels and play them through the rear channels as well? (ie: Left Side and Left Rear would both play the same audio)