I am sorry you faced this. This is really weird and seems to be an isolated incident. I never had such issues. Nevertheless, this should not have happened in the first place.
I strongly recommend you that you get in touch with T-Force in Twitter. They have the super power to take any available measure to get the reimbursement done either as a statement credit or VISA card or whatever you would agree on. Please do contact T-Force before you decide to leave T-Mobile. Just a suggestion from a friend for a friend.
I do remember and I'm bummed you're having so much trouble with it. I see that offer you'd need to purchase both devices on EIP and have the tablet set up with a 2GB or higher Mobile Internet plan. Do you remember if this was done? I agree you should give our T-Force a shot a clearing this up.