Very interesting post, I disagree with almost the entire thing but other than that very interesting.
The tracking error problem that has been brought up over and over is a mathematical problem, not a listening problem if you know what you are doing. I still remember all the tonearm makers that were peddling 9" arms saying how the 12" arms were terrible due to the math, then within a few years they were all making them!!! You know what that means, either the math changed (it didn't), they lied the first time, or they are lying now!!!!!! Either way do you want to buy from people like that?
I have made arms from 9" to 16", all 3D printed and the length that works the best is 12", not because of tracking errors but because the longer the arm the effective mass becomes overly large and sounds wrong with the best cartridges.
Besides, the alignment issue is only a real one for those who cannot set up an arm correctly, it is not that hard just painstaking. You are right about one thing though, the 10" to 10.5" length is the easiest to get the best sound from and is usually the best choice for most listeners, size wise and money wise.
HW