I think setting up and testing will be difficult without a Tableau account. Can you utilize one of the licensed user accounts to get everything setup? You found the right resource for setting this up and this might be best served for you by creating a ticket with Tableau Support http://support.tableau.com
Thanks for the feedback. I assume there isn't a Tableau developer edition that is no cost for testing purposes? Our Tableau users actually use Tableau currently for other products as well, so taking a license would be kind of a last resort.
So, just sat with a user and they were able to login without me doing anything special. The user just needed to click "custom domain" and then type that in, hit continue and it worked as expected. I didn't need to do anything special on the Salesforce admin side.