1- U.S. Treasury Bills mature in 4 weeks, 8 weeks, 13 weeks, 17 weeks, 26 weeks, and 52 weeks. Such a short maturation period makes it an asset that is not sensitive to changes in interest rates (signature bank failure case) and is practically equivalent to cash.
2- tether does not need 100% in reserves. I suspect that 20-40% will be enough to provide sufficient liquidity to maintain the exchange rate of $1 regardless of market circumstances. It must be remembered that probably 10-20% of USDT tokens are probably already lost on dead people's wallets, on lost storage media, on forgotten seeds, or left in the form of dust on thousands of wallets.
3 - In my opinion, there is currently a very small risk of tether problems, and even if there is, due to the tether FUD that has been with us for years, it is already in the price and even this risk is overestimated. Personally, I have heard opinions from people who said "I will invest in crypto after the collapse of Tether and Binance, because the risk is too high now."