Thanks again, Bob,
I know you're trying your hardest to help me figure out how this might be addressed and I certainly appreciate it fully, BUT, I offered Canon test images and the reply I got is that they can't diagnose problems via email, that I had to send the lens, camera and 1.4x to them for evaluation. That was return #3 of 4. The initial repair and the first 3 returns went to Canon's facility in Irvine, CA, the last one went to Jamestown, NJ where they supposedly have "better equipment" after the intercession of Canon's Oregon area sales representative.
There is a happy note to all this nonsense: I didn't have to pay for shipping the last two times, but, that's the only happy thing about the experience.
Since you can borrow a good 600/4, what I'd do would be to take some calibration test shots of a distant target using both AF and MF with a good 600/4 and your 600/4, then I'd give Canon those test shots and ask them to look at them and tell you what they think is wrong with your lens. If you present them with proof of a problem you're much more likely to get it fixed than if you just send in the lens telling them that it isn't sharp.