The guarantee behind the Traditional Account is the financial health of TIAA itself. From your post I don't think you are looking for an elaboration on that as you seem to be focused on Gross and Pimco and the takeover of Fannie and Freddie. I only know what I read about that.
However, if your concern is with Mortgage Backed Securities, remember that the great majority of people with mortgages are paying them regularly and are not in default or behind in their payments. I certainly am not and if you have one, I doubt you are. We are the norm.
If you put your money only in bank accounts or CDs backed by the FDIC you are subjecting your self to inflation risk and that is a very serious and potentially fatal risk. Right now the inflation fear seems to have faded a bit with the decline in commodities but never fear, it will be back at some time.
What I have done is concentrate my investments - other than an inheritance account - in fixed income and that is more than 90% investments in the TIAA Traditional Account. Do I have any fear about it? No, none.
But if you do, and you have trouble sleeping with your fear, then the FDIC backed bank accounts are probably where you should be. But be warned, unless you have some combination of a great deal of money in banks and a short life expectancy, you are taking a long term risk that is very severe - inflation. That I would not do. Rather than doing that, I am depending on TIAA's guarantee in the Traditional Account.
Ray