Re: P90 in middle position for SG build?
Those are legitimate questions. Whether a middle HB or P-90 gets in the way of your picking and strumming, depends on your playing habits. For some players it definitely does. You've developed habits over the years, and if you want a large PU in a place where there didn't used to be one, you may have to change how you use your right hand... or stick with guitars you're used to.
I have an SG with a bridge HB and HB-sized neck P-90, which is a great combination. I get the fuller sound of a HB bridge, and the clarity and high-end of a P-90 neck.
The reason there are very few HHH or H/P-90/H guitars isn't because of a conspiracy or the unresponsiveness on the part of guitar makers. The vast majority of players don't want them, because a big middle PU gets in the way of their right hand, and they really doesn't often more in the way of tones than you can get from a two PU guitar, especially using a push-pull or two. I've had some HHH & H/P-90/H guitars (seemed like a good idea at the time) and have sold most of them. Didn't give me anything different tonally, and my pick keeps hitting the middle PU. They're much better on paper than in practice.