I too went thru the same issues with tech support. I gave them tons of stuff, they asked for more. I said NO. I declined to waste anymore time with them. I even showed them the DNS issue and spoon fed Apple the problem. Its still broken.
I wish people would realize it's not their computer nor their choice of browser. Until Apple corrects the CNAME -to- FQDN entry that's borked, this will continue to fail...
Nothing else and I mean nothing else will fix this.
As always I have just recommended to people to just find another cloud storage provider that actually works. There really isnt anything super special about iCloud Drive. There are other options. Google drive works or Dropbox or even OneDrive.
Just remember to fully encrypt ANYTHING you put on a cloud server, regardless of whom the provider is.
Even Apple can view/read/download anything you put into iCloud or iCloud Drive. They hold our decryption keys unless you encrypt it on your own prior to uploading.
I opened a total of 3 tickets and spent hours with Apple engineering and even after showing them via screen share EXACTLY where the problem is, they don't seem to care or have no one left that can fix this simple issue.
Its beyond frustrating. I just dont know anyone internally at Apple to chase this down with..