Try this: on the grey buttons on the left, click Home, then the Home subheading, and have a look in the bottom right cell in the 'WAN Information' table. Is the correct phone number in that location? As to the number affecting anything, all the usage is counted on T-Mobile's side and the number displayed on that admin page won't be a factor.
As for the surprise usage, the best thing you can do is to monitor it.
First, make sure the Connected Devices indicator on the hotspot display shows a 2, as I'm assuming the two iPads are the only devices you have hooked up to the WiFi hotspot.
Second, find a good data monitoring app in the App Store and install it on both iPads. Make sure it will give you a specific breakdown of data use by program, that way you'll be able to track down what's using all that data and put a stop to it.