I'm from the accounting side and I think we can never be 100% certain that tether is and will remain backed by USD.

And besides the scam/fraud possibility there are other risks. About scam/fraud you already mentioned someone could take the money tomorrow and in the beginning no one will know that.

But I also wonder how will they pay any bill? Or are there really no costs to keep tether working?
(Sorry I really don't know anything about that coin)

I think I would say the credit risk (I think that's how we call this) is the biggest risk. Let's assume they have this 2 billion divided over 5 banks. That's an average of 400 million per bank. If one bank goes bankrupt the whole 400mln will be at risk. Most likely the biggest part of the amount will be refunded, but that could take years. And what happens if that causes is to lose fade in tether?