Suppose someone said that world population is increasing by one billion per decade. And then you said, that's impossible, because the population would have been negative in 1900... All that "argument" shows is that the trend in question couldn't be true across all time; it doesn't refute the contention that that is what's happening now. In the same way, what you just said only shows that doubling the CO2 can't lead to 3 degrees increase for all possible concentrations of CO2. It has no bearing on whether this is true for the range of concentrations that matter to us.

Half the amount of CO2 10 times! Then it will be less than 1ppm CO2 in the atmosphere. Do you expect 30 degrees cooling?

But in fact that is the expected result. "... the planet's effective temperature ... is about −18 °C, about 33°C below the actual surface temperature of about 14 °C. The mechanism that produces this difference between the actual surface temperature and the effective temperature is due to the atmosphere and is known as the greenhouse effect." CO2 is directly responsible for only about a third of this, water vapor does most of the job, but the water vapor is the dependent variable.

Reality is complicated because water vapor causes cooling (as clouds) as well as warming, and there is room to argue about how these effects vary, but the arguments you present aren't enough.