While I agree with your summary insofar as Government action is concerned, I think you place far too little blame on the huge financial houses, who were more than happy to operate in the environment that the government created. You make it sound like a matter of, "yeah, there were a few bad apples, but..." But the incredible magnitude of the failures in question make it pretty clear that there were a whole LOT of bad apples, making a whole lot of really stinky deals.
Lehman Brothers, Countrywide, and all those others were not forced by the government to get in so incredibly deep as they did. They were busy making (illusory) big bucks on shady deals, and eventually the false basis of their financial arrangements came home to roost.
FWIW, I think you probably need to add another contributing factor; namely, the changes in banking regulations allowing multi-state mergers, letting banks deal in securities, and so on.
One of the major reasons why this crisis is so huge, is because of the significant concentration of assets within a quite small number of financial institutions that have been created over the years by bank mergers and acquisitions. Once those huge banks began engaging in the highly questionable financial arrangements that have now crashed, it was only a matter of time before a too-huge-to-allow type of failure occurred.
As for the commercial banks and investment banks, they did what they've done for decades - put the safest, most liquid securities onto their balance sheets - i.e. mortgage backed securities. You can argue that the leverage ratios should not have been allowed to increase (i.e. $1 of assets supporting "X" amount of borrowing). However, the point of the collapse is that, due to the high concentration of sub-prime mortgages and the accelerating default rates, what was a very safe, highly liquid security now has no value and no market.