That has to be the longest, most time consuming rant I have ever read.
It seems your area has a 50% hit or miss rate, judging by general reviews.
Just wondered if you took another tack and see what happens? You did the router replacement, have you actually (Recently) checked line noise ratio's, checked all physical connection points [cable and RJ45 lines] you can, including your PC and it's ability to perform correctly, software and hardware? Did you wave the magic wand in the right direction (Just kidding.) Try a friends laptop, something that's already proven on another cable connect.
If that all fails, become a Comcast technician, part time and have a Comcast party, with some solid techs in the mix.
I'm patient, but you have taken that to a whole new dimension, or too stubborn to have some local-outside-of-Comcast help. Doing that would give you optimum credence in your complaint, to Comcast.