1, Can you hear 20000hz? If answer is no then nothing is wrong with measuring up to 10khz for THD.That "industry standard testing protocol" was made to draw 0,00001% THD and 5-100000 khz. In other words that protocol was implemented to sell crap.
Yes, it is a way to compare different amps. But it's a compete ignorance to judge how those amp will work on real load.
Sure it can. Take any 2-3 way speaker with more or less complex crossover and such load can simulate every situation. You all forget also that any speaker is also a generator (press on your woofer by your hand while it is connected to multimeter and you'll see it generates something). And when such generator is powered by amp with deep negative feedback it all cause very high distortion (sorry, my english is limited. I don't know all terms).
And when we're measuring even on such load as topicstarter made we don't see that effect.
2, Negative feedback only improves immunity of back EMF not the other way around. Again, need proof.