Finally! After months of trying to contact Amazon, the IRS (whose "international" numbers tend not to work in Canada), an American tax lawyer, and anyone else I could think of who might know how I can get my Amazon book-selling tax issues handled, I finally found a person who seems to know what he's doing.
He needs to do some research before he can firmly recommend the most applicable and cost-efficient way to go about getting the documentation I need to continue selling books to America, but at least when I explained my problem he knew what I was talking about (the other pros I contacted didn't).
If you're a Canadian who sells books on Amazon.com and doesn't have an ITIN, SSN, or any of those other collections of capital letters that you need to prevent Amazon from pulling your books out of KDP, be sure to follow my blog, as I'll be posting the solution once it's been hashed out.
In the meantime, don't worry; I don't plan to turn this into the Everything About Taxes blog. I have more entertaining things to write about. :) But after all the hassle this matter has caused me, I'd love it if I could spare some of my fellow authors the same frustration.