I agree with your take that the impact on America could have been much smaller in all sorts of scenarios, including just taking it more seriously sooner (like during the time frame that you're recounting).
I'm not sure that the travel ban from China in particular would have significantly affected the course of the pandemic in the U.S. We now know that COVID is very often totally asymptomatic and that it had already spread to many countries (possibly including the U.S.) by January of last year. People arriving from countries other than China could easily have introduced it into the U.S., and very possibly did (and certainly have many times since then). The U.S. didn't have the testing or tracing or quarantine capabilities to control the transmission of the epidemic in the community.
This skepticism is not meant to take away from your other points, which all seem right to me. I think many other kinds of responses (or awareness or understanding on the part of the American public) could have had a big effect last January or February. The responses, awareness, and understanding we actually got were terrible.
