Sure, any channel can at a given instant be the loudest one. That's why amplifiers have equal power capabilities in each of their channels.
A 6dB loss per doubling of distance is correct for an omni-directional sound source in anechoic conditions. As mentioned above, the result in actual home listening rooms with reflections added to the direct sound is about 3dB per doubling.
Anechoic sensitivity ratings are used, since the 3dB figure takes into account the room contribution, and it shouldn't be counted twice.
Rooms aren't "filled" with sound at an equal level, despite the term being often used. The cathedral ceiling has a small effect on the level of room contribution from reflections. There's no easy way to determine how small the effect is, other than actually measuring with an SPL meter in your room at various distances to determine how close the loss in level is to the nominal 3dB per doubling figure.
