The difference between t-bills/notes/bonds is the maturity period, the frequency in which they are offered for sale, and of course their interest rate. Bonds have 30-year terms (no longer available), Notes are offered in 10, 5, and 2-year terms, and Bills are offered in 52, 26, and 13-week terms.
The difference between a 13-week T-Bill vs. a 52-week T-Bill is the interest rate. Generally speaking the longer the term, the higher the interest rate that you lock in. The disadvantage, of course, is that you are locked in for that period.
Yes, you can liquidate any Bill/Note/Bond early. There is a bidding process and centralized marketplace where these are sold on the secondary market. You will get close to what the interest accrued has been, but there is a fee that applies. It's not that expensive if you sell directly ($34). If you go through your futures broker they may charge anywhere from $35-$100. (Consider if you had bought a T-Bill for $9750 with face value of $10,000 - if you liquidate after 1 month you may get ~$9780 for it buy pay liquidation fee. Also, some brokers will rollover your T-Bills for no cost whereas if you liquidate and buy new T-Bills at later date you pay more fees. )
The beauty of a T-Bill/Note/Bond is that it is a 100% electronic instrument that can be bought/sold/transfered at any time. It is not like a savings Bond which is a paper asset specifically in someone's name.
Only T-Bills are sold at discount, Bonds and Notes are not. T-Bills are preferred over Notes & Bonds because being short-term they are much more liquid, meaning you'll get closer to the true value of interest accrued if you have to liquidate prior to maturity. Interest on Notes and Bonds are also paid semi-annually, so you have a skewed curve every 6-months on valuation of Notes and Bonds on the secondary market.
My guess is that your broker was assuming you meant "T-Bill" although some brokers due accept "T-Bonds". They usually have a % of how much of your Bond they will accept as margin.