If we're going for some armchair philosophy (with added tinfoil hat) - I agree with all your points. I'll also claim that language developed from vocalisations

If we're going for some armchair philosophy (with added tinfoil hat) - I agree with all your points. I'll also claim that language developed from vocalisations because humans are excellent at mimicking the sounds they hear around them, and they developed this mimicking ability mainly to impress members of the other sex and, well, the rest is history. Totally unprovable, of course.