Steve, I haven't been following this closely because I'm not very interested in DIY in general or class D amplifiers in particular, but I've now looked at the last couple pages. First point, as we've discussed at some length in the past, is that although two 8ohm loads driven in parallel result in a 4ohm net load, that isn't what's happening in a "4ohm" speaker. Say, at 100Hz if the impedance of a bass driver is actually 4ohms, because of the effect of the crossover circuitry the impedance of a "4ohm" tweeter section at that frequency might be more like 100ohms. The resulting impedance at 100Hz would then be 1/4+1/100=1/x; 25/100+1/100=1/x; 26/100=1/x; 26x=100; x=3.85ohms. So no, a "4ohm" speaker(which the M80s aren't at most frequencies, of course)isn't the result of two 8ohm sections being driven in parallel.
You ask "Am I missing something?" on the bi-amp question. Yes; although you refer to "each amp", there's only one in a two channel amplifier with one power supply section. The output transistors in each channel have no power of their own, and can't supply added "headroom".
Also, note on the voltage splitting question that two voltages in parallel don't form a voltage divider(series connection does that), and the output voltage to each load hasn't been cut in half.
