Overture - Yes this will work just fine. Connect the two via ML, and you will have complete control over the Overture as if it's part of the BeoVision. I doubt the BV7's DVD player is better than the Overture's though. Maybe you could do a comparison for us? I haven't done anything like that.
Airport Express/optical. No, this will not work. The BeoVision 7 does not take stereo PCM digital signals, kind of irritatingly. You could always use an external DAC with the Airport Express though, and connect that to one of the BV7's inputs.
Turntable - Yes, you can do this. No, there are loads of line in ports, you could even use a scart input as an audio input. My suggestion would be to use the Overture's AUX input though, as you'll still be able to get this up over BeoLink.
As for choosing a source, it'll be V.AUX, SAT or whatever for stuff connected to the TV, and CD to choose the overture's CD player, Tape to choose its tape player, and A.AUX to choose its AUX input. It'll all be selectable via the Beo4 though, don't worry. It's perfectly integrated.
Speakers - do you mean, when watching TV you want sound from all speakers, and when listening to CD you want it to automatically switch to speaker 2? This can be done, however when listening to a source connected directly into the TV (instead of part of the overture) you'll need to choose speaker 2 manually (speaker 2 is when only the left and right hand speakers play, and a subwoofer if you have one). You'll be able to choose which speakers sound easily, via the remote control without having to fiddle with menus - don't worry.