With the way these multi-trillion dollar companies operate today and how the support staff act like corporate drones, I have no hope that attaching PGP signatures would lead to any different outcome. The response from the support would probably be going to like, "PGP what?"

How does a PGP signature help? The person who stole your work can trivially strip your signature and add their own.

If every copy in circulation has your signature on the copyright page ...

(This proves that you, the holder of the private key, are the author, not someone who goes by the same name)

Anyway, if you still think this won't help you can use a notary service, yes.

I don't think any more human interaction was involved than a very bored and harried call center employee clicking 'confirm' on a bot-generated response here. You can have all the cryptographic proof you want, but you're screaming into the void.