I am reading all of this and understand the frustration. I did the same thing for MONTHS chasing the sponge demon. Spent way too much time on it only to find out it was the brake booster push rod. I don't have the sports car level of feel but I am more than happy with them.
Here I don't see that it has been checked but, for something so easy to do, I would rule that out before removing any safety equipment or spending any more money. Worst case scenario, you over tighten it and have to back it off a few turns. Just sayin