Q: Does this prompt come up only once and the user won't see any other prompts regarding email, right?
Yes. The ID is persistent once generated, unless you manually change it afterwards in the Host settings. Therefore there is no need to send email each time the program is run and notify the user about that.
Q: Why can I not access the documents folder?
When you open the File Transfer window, in the Access menu
try to switch from "Current user" to "System".
3) I'll be using RU for one host and one client. I have activated the free license on the client, but 30 days trial notification is still on. Do I need to buy a license for one host and client?
You need to restart/reopen the Viewer.