Tether didn't prove anything other than they have $1.8 billion in an account. There is no guarantee from the bank that the money will stay in the account for any length of time or where it came from. It could be a short term loan for all we know. There is also no guarantee that the money is not encumbered by some other financial derivative.
I have not thought about it. I do not know the legal aspects of derivatives of that jurisdiction, but that seems to me to be more of a bank matter than Tether. In the letter, the bank states that the portfolio is equivalent to such a value, but does not discriminate the formation of this portfolio.
Could this portfolio include other non-cash assets? Or maybe loans?