Is Bass Non-Directional In A Two-Channel Rig?

Yes, assuming that levels are matched, you have a 4th order electrical low-pass filter, and your sub-woofer is free of distortion, port noise, and rattles

One caveat is that stereo bass with phase differences between the two ears can create a sense of envelopment although you can't localize it.

The counter argument to that is that random, asymmetric placement of multiple sub-woofers produces more uniform frequency response above the room's fundamental resonance and avoids one-note bass problems.

Very low frequencies like 40Hz and below start to become omnidirectional but their harmonic content doesn't, and this makes many woofers localize. I can always tell where a subwoofer is in someones room. I recommend stereo subs unless they are crossed 30Hz or lower.

"Irrelevant. That has to do with sound absorption issues."

Semi-correct, actually a bit more about wave length and dispersion......

as a P.S. to my last response, you might through a little waveguide theory in there as well....