MMM Looking at your designs I'd do it different, and if I was 'doing it', I'd get a projector.
I'd do this:
Projector on same wall as door, projecting towards window. You could probalby make/buy a sweet pelmet setup to hold curtains and the screen discreetly. As you noted, you can't use the window and the projector at the same time, doing it this way solves the problem.
Orienting the room like that solves also the problem of the door/sofa.
You can put the surrounds above the door pointing down, this hides them away, personally I think surrounds stuck on the wall as they often are looks pretty tacky, this would be almost the perfect spot for them in terms of you'd pretty much not see them. (when looking into the room or being in the room.)
Now as I see it there is 3 or 4 choices to make now. (with IMO in comments)
A) Sofa on wall beside door, desk in front of window (blocks radiator, veiwing angle on screen may be compromised with low down sofa and front edge of what looks like a substantial desk, plus it would just be weird to be watching a film with a BF desk in beween ou and the screen.)
B) Sofa on wall beside door desk at right hand side (where you have sofa just now) - (could work, desk might interfere with screen but should prpbaly clear it, and should clear the door as well - possible option. desk on other side ouldn't work so well it ould be in front of sofa.
c)Desk on same wall as door, and sofa in the middle of room so it would go |wall|desk|chair|space|sofa|space|screen|wall| ( May or may not be enough room for that. ) 3 metres, could be a bit tight)
d) do you really need a sofa? Desk on same wall as door, and a decent recliner chair or 2 with foot stools, and a nice coffee table thing in the middle of the room or bean bag or something.
0.02
pic:
sofa can be bigger or have a table
black thing is book shelve, dvd storage, about 2 feet high, or maybe taller?
thin black thing on right is screen
no room for large speakers, so probably a sub+sat setup?