Ho, that's a different kind of ISOT, one I'm not familiar with.
But...if a medieval level tech is already present from before 8000 years ago, one would think that they'd already long develop nuclear weapons since
, and the coming Native American civilization would have been immediately absorbed into it. Heh, I can even picture America doing colonization of Europe instead of the other way around
EDIT: OK, let me contribute a bit. You can make the Middle-Earthian suddenly appear in the area at the time of interest. Though, I can't think of a justification of why they would fight the colonial power on the side of the Natives, unless of course the Europeans attack them first (which is very possible I guess). Or, you could let them appear a few centuries earlier and resolve the things inside first so it'll be them instead that the colonial power encounters. Oh, btw don't forget about germs! Arguably, the biggest killer the colonial powers brought to the natives weren't firearms nor swords, it's their diseases, which they spread by simply being there (similar to how the Mongol presumably brought black death to Europe).