The real-time balance always screwed up if I have positions over night. the real-time Net Liq is not reliable.
I open and closed a position overnight, and my balance is showing about $100 less than what it should be. So its not just u.
They will buy the T-bills for you - just tell them how many dollars and maturity. Just email them for details. I'm not sure if they have a minimum acct. balance for this or not. Keep in mind you need to keep enough cash (not in T-bills) in your account to handle any drawdowns in your trading, otherwise they will have to sell some of your T-bills unless you deposit more cash.
It always screwed up if you have open positions during their system reset time around midnight. Amount could be huge. I have contacted them for a long time. Somehow it has never been fixed. I don't know why.