

If you were using the default port 25565, you could simply have a DNS A record pointing to the server IP. But since that is not the case, you have to additionally set up a SRV record in your DNS. NOIP describes what this is and how to set it up on their service here, but it will of course differ for your DNS provider.
This still requires that the Minecraft server port is directly accessible from the other clients, but it sounds like that is not the problem
It is a public GPG key (not a Microsoft Publisher file) and while I don’t know, I assume it is used to verify that future updates are from Nvidia and not a malicious third party. It is really common (if not required) to install and trust a new gpg key when adding a new repository in Linux package managers, and done for safety reasons without privacy implications, and I am guessing this is what Nvidia is doing here.
Where do you see any indication that it is used for identification of you/your machine?