Highly Distorted Output of Class B Power Amplifier
I didn't really read the whole question (too long, and didn't get to a clear point fast enough), but this is not what you seem to mean by "distortion":



This is very clearly the amplifier oscillating on its own when given a little kick by the input signal.

A brief look at the schematic shows why this shouldn't be a surprise.  There is no capacitance on the power rails at all!  The output is loading the power rails, which changes their voltage a little.  That small change is picked up by the input stage, and then amplified thru the rest of the amplifier.

To fix this:

Put decent capacitance to ground on each power rail.  This should be several 100 &micro;F at least.

Break the power rails to the left of Q5 for the positive, and the left of Q7 for the negative.  Put a small resistor in series, then followed by another few 100 &micro;F to ground.  This time, though, add some high frequency bypassing too, something like 10 &micro;F ceramic to ground on each power line.



I think I remember telling you this before, but I don't feel like digging around in old history right now.

I think you are also told before that Darlingtons are not a good idea here.  They require higher drive voltage than output, and have rather high saturation voltage.  There are better ways, like using a power PNP for the top pass element of the final stage, then a power NPN for the bottom.  Those can be driven by smaller NPN and PNP transistors, respectively.  However, that is beyond the scope of this question.