the experimental framework for finding out absorption spectra goes back like 100 years and is basically high school level science. not only that, we have quantum chemistry models that predict empirical results from atomic configurations that are 20+ years old and match that empirical data to a very high level of accurany. there is plenty of ambiguity to attack in climate modeling, but you chose the most settled and fundamental thing to poke at.

make the argument explicitly. Here, I'll do it for you: doubling co2 levels should only lead to a 1c increase in temperature (~3w/m2 extra forcing).

That ignores all the other things that happen besides co2 forcing alone.