When you click on the link supplied in the Rainway guide, you will be directed to a Discord sign-up form where you have to enter an username and check the box that says that you agree to the terms of service and privacy policy. I'm not familiar with all the configuration options Discord can offer, but in this case it seems you can't bypass account creation here.
Ah they might've changed it to combat bad actors, I haven't tried to use Discord without an account for a while now so sorry about outdated info
There are several options the 'server' admins can choose from. Some let you do everything without account, some only let you read, and others require an account for everything.
I think you are right. You can still have one off sessions but maybe depends on the privacy settings of the server.
You can view the server, but you can't write anything, and you can't use reactions, so you will actually need an account to open a ticket.