though, I’m no economist, but what happens if Fannie and Freddie DO collapse and fall? Does that bring down the US economy? Anyone have a clear picture that they can tell me?
They can't collapse and fail anymore, they already have - as "private" entities - as a result they were seized and taken over by the U.S. government due to "implicit" guarantees they were operating since inception.
They are now responsible for about 97% of newsly issued mortgages... Ideally they will be wound down over time, without new programs replacing them, though Barney Frank and other Dems are trying to "reinvent" them or replace them with something similar, different in name only.