Hifitime, that was pretty ingenious using those o-rings. They do add weight, so the markings might not be as accurate. I think if you keep the interconnects, speaker wire, etc. straight, you can figure out which channel comes out of which speaker. My setup and age(loss of brainpower-manifested by inability to do complex thinking. Complex thinking is more than one thought at a time.)has stopped me from keeping the channels straight, but I used to do it that way. Remember, the record is moved in relation to the stylus(I'm not sure that matters, but we must realize what is going on.) I do believe that anti-skate traditionally is related to what VTF is used. I usually set the VTF before fine-tuning the anti-skate, but whatever works. It can't ever hurt to check out things technically, like you did at your dealers. Your theory about the center of the record that has good high frequencies on both channels sounds right, but remember to not get too hung up on theories(or at least trust your ears). I think you are correct about the end of the record presenting the most problems. In fact, some of the more recent lp12 reviews complain about this, but I believe the reviewer has incorrect anti-skate. I do think that studies back up the fact that the inner grooves have more distortion.
