Actually the fed only influences the short term interest rate. Mortgage is mostly tied to long term interest rates such as 10 year treasure yield.
China's massive buying of US treasures and other US bonds helped a great deal of creating and maintaining the low long term interest environment in the US.
3. Investment bankers within IB does not strucuture or securitize the CDOs, it is done by Fixed income specialists in ABS/MBS area. The famous F.A.S.T. division in Bear Stearns & Co is a good example.
