4. According to BMXTRIX, the screen will be wavy in about a year. However, if it works for you, the High Power screens such as made by DaLite are much stiffer and more resistant to waving. And even with waves, they disappear much better than with others. I use a 159" pull down manual HP dalite.
Waves are a given, but if you need a retractable screen and are not willing to invest in a tab-tensioned screen at this time - which will never develop waves - then get a far less expensive manual screen. Not sure of the price of the DaLite High Power manual 159" screen, but at just a few hundred bucks or so, it is the best alternative to doing it 'right'.
A receiver with dual HDMI outputs is likely not realistic, and certainly isn't necessary since you need the same source to both locations.
Instead use something like this...
http://www.monoprice.com/products/p...=10113&cs_id=1011301&p_id=4921&seq=1&format=2
Take the output of your receiver to it, then to the two displays.
For audio, upgrades, there is no reason not to use the speakers which are built in. Have no doubts that for the money that is spent on floor standing speakers you often get better sound than in-walls, but that doesn't mean in-walls are inherently bad or of poor quality.
These speakers are from a reputable manufacturer and are of mid-level quality, which I would consider adequate for the room and the budget you have - especially as surrounds, though the configuration of the rear center channel is more of a 6.1 setup location vs. a 7.1 setup location. I would likely try to get the rear center channel moved over to a proper 7.1 location and add one more matching speaker, then invest more money into a good center, L&R main speakers.
For the center channel, you just need to mount it under the 52" display at a point where it isn't covered by the screen.
I am currently doing this in my home with a 106" screen and a 60" plasma with a DefTech Mythos 8 speaker located on the wall immediately below the plasma and the screen stopping right near the bottom edge of the plasma.
Had I to do again, I likely would lower the screen a bit.