A. I don't think anyone really knows with 100% uncertainty what the deal was. If I had to guess I would say its only a firmware difference, but it's hard to say which anecdotal reports you read on the internet are the most accurate ones...
B. If you have to retrograde your firmware in order to see if there was a possible defect in your screen that only really shows up with old firmware, then you really shouldn't bother.
You can, however, compare and obsess about your iPod Touch as it works now, in comparison to other iPhones and iPods. My screen still has a small amount of shimmer in the blacks, but it's near unnoticeable and overall the screen is GREAT quality (color, sharness, overall 'richness'.) If I ever notice that other iPods look better then I will perhaps study it a bit more, otherwise I'm not going to worry about it. I already really enjoy watching movies on my Touch (which wasn't the case with my Week 37.)
I'm more afraid of exchanging it and getting a worse screen (stuck pixels, etc.) than trying to upgrade a 95-98% screen to 100%.