Ethan, thank you for providing such clear and concise documentation about Room Setup. And I'm glad you didn't mind me pasting links and pics of your articles in here. I was going to draw up something for the reflections, but I knew your site had good information already.
Roadking00, about sitting too close... Ethan's right, well at least for a 65" plasma. It sounds like you can't really move too much to optimize viewing, so I would look into optimized viewing distances for screen sizes, then opt for a larger screen : http://www.avsforum.com/t/1004723/chart-distance-x-screen-size-standards-smpte-and-thx
I personally just chose what I was comfortable and my preference. I base it on where I like to sit in a large theater (typically in the center axis and just behind half way back), then just scaled it down for my home theater. My rough guide is based on when I sit, my legs naturally rest at some angle apart. Basically I am 6'2", and when sitting, my knees are typically about 18" apart. Then I follow the straight line projection from my legs, and sit where the edges of the screen line up with those leg-lines. It turns out I am basically near the THX min viewing angle and near the max in the second row and first row respectively. Meeting THX standard was never my intention or target, but it just happens to be matching quite closely. Maybe I should name my method the TK Standard...
As far as speaker placement, there is an art to it, and every room and equipment setup can be different. Experimenting can be half the fun of it. There are some basic guidelines such as distance from front walls and side walls and angle toward listening position, but with your setup (the window and fireplace [reflections] and entrance up front [absence of reflections]), it's going to be just as good to just temporarily place them in several spots and listen to each one very carefully. If you can't hear any difference, then placement won't matter too much, but if you can, then choose the one that sounds best and add what absorption/diffusion you need, if you can.
I have a 17' x 21' room, with the screen just about 12" away from the front wall. My L/R speakers (8.25" W by 17" D) are placed so their rears are about 12" to 14" away from the front wall. I have them toed in so the axis of each speaker meets just behind my head at my main listening position center of second row, which is about 11' from the screen. They are placed a few inches away from the edges of my 100" diagonal 16:9 screen. I played with the placement, a lot before settling on the arrangement just described. I had them against the wall, much farther out from the front wall, straight, toed in, really wide, closer to the center, any combination of these. I had a friend over listening to some music about a year ago after I had optimized for my liking, and he kept asking if my center channel was on. Of course I knew it wasn't because that amp wasn't even turned on. However, by his repeated questioning, I took it as a compliment that my placement and angles had been just right.
Obviously you are more limited by using in-wall speakers, so you will have to just live with some compromise, but you can make it sound the best that it can by the things suggested above.