This seems to strengthen the argument, supposing these diseases were caused by the microorganisms. This would explain how the microorganisms led to extinction.

Humans' susceptibility to diseases doesn't matter. If the animals were resistant to other species' diseases that would be useful, but this just isn't relevant.

This establishes a correlation between hunting and extinction. This weakens by undermining the essayist's denial that hunting could be the cause of extinction.

This doesn't affect the argument either way—we don't know how common this is, or how it affects the likelihood of extinction by disease.

The argument is just about the animals that became extinct within 2,000 years of humans' arrival, so later extinctions aren't relevant.