I'm not so sure about cloud cover by itself, but surely CO2 is small compared to all other variables combined. The thing, is, all other variables are largely in balance, otherwise we would see much larger ups and downs, especially aside from the trend that corresponds to the CO2 change. The trend that we see does not require larger variables than CO2 and the mostly obvious higher temperature -> more water vapor reinforcement.
Also, the outward radiation at the surface, which corresponds to the black body radiation, is largely in balance with measurable incoming short wave and long wave radiation at the surface, not leaving that much space for additional factors (like adiabetic pressure). (Unless these largely cancel each other out, which would pose the question of why bring them into the climate change discussion in the first place.)
Nevertheless, that's why climate science spends so much effort in creating complex models and simulations: in order to verify all factors that might play a role. And that's why Lindzen, Curry and Spencer do get a certain amount of attention from climate scientists in general. It's just that their desire for political attention is not in relation to the significance of their research compared to climate science as a whole.