[Sephiroth]

Good. To answer your addedum first, the living room AE must not be in Bridge Mode to the SH.

The AE in the studio would presumably be in bridge mode with the AE in the living room. So with that unusual (but obviously reasonable) topology, one does need to move outward from the modem and take measurements (as your addendum states you have done - all the way to the studio AE?).

One thing bothers me about the wireless. You say you've got the same wireless network name. Which physical wireless is being picked up by the MAC AIR? and are the frequencies the same in which case there'll be conflict.