That happened thousands of times (if not millions, hard to estimate) before human race came to existence. How on earth can it be unnatural?
Why is it that people that should know better, because they have all the capacities to, are the worst climato-skeptics ?! Unbelievable.
2. It's not just the speed, humans makes movements possible that were previously unlikely or impossible. Argentine ants are currently wreaking havoc across the globe. Do you think they would have ever found their way from Argentina to Australia and Hawaii without the help of humans?
3.
> contributing to biodiversity lol
Source needed
2. If they are successful as invasive species, then yes most likely that would happen given enough time. Even the most remote islands in the ocean are populated with life forms that are too modern to be explained with plate tectonics. Nature has it's ways (besides evolving humans).
3. As you noticed humans spread all kinds of viruses, insects, blights and other unfortunate beings. Like them or not, they are as valid as a part of the biosphere as the chestnut tree.