When animals invade without the "help" of humans, it happens in a very different speed which the ecosystem can adapt to.
And even tho I made the joke, I wouldn't say humans are the problem. Humans lived more or less "in harmony" with nature and then came colonialism and capitalism. Humans are capable of living differently, the market is not.
Yes, but never on an ecosystem endangering scale. Species go extinct, that's what happens in nature all the time and humans were the cause in history and prehistory. But never on the scale of today.
If you have the mega fauna in mind: it's very debated how big of an impact the humans had. There are different factors at play for sure, like changing climate. Was it really humanity that made the woollen mammoth go extinct at the end of the ice age?