The only theory that comes even close to explaining the boom is that low rates in Treasuries, as a by-product of low FF rates (although I don't buy into it since they are different products), resulted in yield-chasing to RMBS and other products. This is a silly argument as long-term rates aren't set by the Fed, FF rates have limited affect on LT rates when a bubble is in full blown frenzy mode.
As far as the GSEs, the guaranty was always implied but never explicit. The GSE's proportion of the mortgage market actually DECLINED during the bubble.
Everything you said about option-arms and liar loans is true, but it was all connected due to yield chasing resulting from low interest rates. The argument is circular -- there was huge incentive to keep pumping out mortgage product to securitize and sell to investors, who were chasing yield...again, all because interest rates were TOO LOW.
This caused all kinds of bubbles. Housing going up 15%/year for 5 years straight only incentivized the public to further reach for overvalued real-estate, which lenders were all too happy to provide b/c they could lay off the risk to securitizers who packaged the crap, sliced it and diced it into CDOs and sold it to yield-chasing institutional investors.
No one can convince me otherwise that the Fed and its low rates/easy money policies are most responsible for the housing and credit bubbles that took down our economy.