1/ If by that it's meant speaker variations tend to have a greater influence in a macro sense than many other parts of a system, I'd probably agree, assuming the rest is set up & working well.
2/ No customers; as I made clear that speaker was designed as a favour, but yes, it had a specific set of goals. I don't personally get much pleasure from random dabbling; no issue with it, different strokes for different folks, but I'm lousy at construction, and prefer the 'design' & listening part. Re a 3rd or 4th order on a ribbon -rarely. The two speakers Colin & I have with ribbons for e.g. use low Q 2nd order filters. I would never use less than that with a small ribbon / planar variation at relatively modest XO frequencies -partly to ensure an ample margin of power-handling, but also because of the piercing distortion almost all of them possess toward the bottom of their range.
The ideal from my perspective, contrary to what is assumed from certain quarters, is to use the lowest order acoustic slopes practical for the intended use and goals. You need to factor the electrical order into that as well; I rarely use 1st order high pass filters at all < about 10KHz simply because they cannot prevent driver excursion from increasing below the XO frequency, and that rarely does performance many favours in my experience -I can usually hear the tweeter protesting, unless it gets help from other quarters, which usually means more components. On the other hand, I'm rather partial to a low Q 2nd order, especially if you work it so you obtain what amounts to a 6dB/octave slope at the XO frequency & for perhaps an octave below before increasing to a higher order acoustic rolloff. All of which is of course assuming it is actually practical to use low order XO slopes with the drivers employed, which is not always the case.
Re phase rotation, to the best of my knowledge it is not universally accepted (far from it) that the phase rotation at the XO frequency / transition band of higher order filters has such catastrophic effects on female vocal tone as you state. I can think of plenty of speakers with higher order XOs that sound superb on female vocals. Depends exactly what you're doing and how, and the listener. Personally I prefer the shallowest slopes practical; when they are not practical, I try to compensate via other means if necessary. That's just part and parcel of design, and one of the most interesting aspects of it for me. We're all picking from compromises -if I see one more advert marked 'designed without compromise...'
![Wink ;)](./images/smilies/icon_wink.gif)