I think we are talking about two different programs. I am talking about this program
here Not Amazon Cloud Drive which no longer has anything to do with music it's just for Docs and Videos.
I just open the program and choose import to cloud it takes me to my Cloud player on Firefox then I choose import. It's really simple use.
Alot of my music is from over the years before everything went digital. I rip them from CDs I have several artists that aren't available digitally and never will be.
I use MP3tag to fix up problem songs. For Amazon to recognize their purchases from older years you have to make sure the comment filed in the tag is still there with the songs Amazon ID.
Amazon wont try to match anything 320 bitrate or higher. Seeing their music is 256kbps which is okay on some Albums not so okay on others.
I don't know why you can't copy music from your PC to HDX I can without any issues.