Roger Cicala of lensrentals has dealt with Canon repair a lot, as well as with other repair, for problems that lensrentals staff haven't yet learned to fix in house. He has a VERY SIMPLE SUGGESTION. If you go to the trouble of testing your lens with FoCal or with a home-brew test (even the "brick wall"), you need to send the test photos and the test methods that you used along with the lens, and a list of the defects that you found (eg. "lower left corner consistently soft, see photos #1,3,4..."). Load up your images and text on a $5 thumb drive labelled with your lens SN and your name, and ship it in the box with the lens.
For more details, see his lensrentals blog. But really, this is common sense, service personnel appreciate being shown the specifics of the user's issue with the product, and are likely to look harder when diagnosing the lens's problem - human nature. A lot of people return products with complaints of "it doesn't look sharp" but there may be unrealistic expectations or the product is defective under some conditions but not others, and it is hard to ID some problems in a quick inspection.
I have done that, only I burnt the images to CD, my experience is they don't even look at them, they sent the disc back unopened.
I believe they put the lens on a reference body or rig and draw their own conclusions from the results from those results and that alone.
As for the OP, I did write a rather blunt reply early on in the thread but it got taken down, naughty me! The crux of it though was that these things are tools, they are not hermetically sealed pieces of jewelry. They get dust in them, they get scratches and they end up needing servicing, that is the nature of tools. They are not collectables and they are not precious, they demand to be used as the makers intended, regularly and without compromise, if you do that then you will not only get the best out of them, but you will get dust in them and scratches on them.