The activation process was a bit confusing and the two store staffers who helped me (including a manager) weren't on the same page all the time.
I did bring my iPad to the store, and I'm glad I did, because they asked to verify that I was connecting to the Internet (by viewing a web page).
Ultimately, I ended up paying the ten dollars to activate the T-Mobile SIM card that had shipped in my Retina iPad mini. They did not bust out a new SIM. The store visit was about fifteen minutes, and when I left, I had an iPad on the T-Mobile LTE network. Mission accomplished.
The sheet of paper they gave to me shows that the device is on the 200MB free data plan, so hopefully when the next reset day comes up, the meter will just reset and I won't get charged/pestered about a data plan renewal.
I can also confirm that this T-Mobile iPad data service is compatible with Onavo Extend (a cellular data compression service which runs as a VPN). 200MB is a pretty modest cellular data quota for a tablet, so I'm hoping to squeeze out a bit more using Onavo.