It does seem that everyone here is using the words "could have' judiciously. And of course anything was possible. Not necessarily coming to conclusions yet.
Pat- I have spent a lot of time with guys on boats, and your observation of what typically happens is spot on. However it is interesting to note that the first "successful" settlement of the new world didn't occur until European diseases took a huge toll on the Native Americans. There are reports of many large empty settlements, decimated by epidemic. Of course they would be on the better sites, and the Europeans moved right in at a staggering rate after 1620. Seems that the vikings complained about the "skraelings" dissuading them from their endeavors. What do you think might have happened to the half European children?
I had not heard that about the range of the wild grapes, it opens up the possibilities even further. It is certainly easier to speculate about what might have been, than to defend assertations of what didn't happen, as commonly happens when some of the folks who make a living at this stuff sometimes are prone to do. Are you sure it wasn't martians, Tony?