The difference in volumes of CO2 for a 20 F difference is closer to 0.3. As the beer/bottle cools, the pressure inside the bottle also goes down, so the retained CO2 is not strictly a temperature effect, but a temperature and pressure effect. Agreed, for the same bottle, the colder one will retain more CO2 in solution, and my 30 minutes in the freezer is insufficient to re-absorb that difference in CO2 back into the liquid.
Bottling vs kegging: As you noted, when the keg is pressurized it takes time for the liquid and gas to get to equilibrium. But with bottling, the CO2 is created in solution, and comes out of solution into the headspace. The two carbonation processes are different. In bottling, the liquid is saturated with CO2 before it moves to the headspace, with kegging it moves the other way around.