(a)
Local diseases killed many Europeans. Large numbers of Europeans settlers died when they arrived in the Americas because they could not adapt to the local conditions.
In this case, Europeans =/= Norse. The Europeans in question originated from the UK, in some cases from cities with limited farming and craft skills. And managed to end up resorting to cannibalism during one of the milder winters. Also, the Spanish who arrived in a climate totally different from their usual one.
The Norse came from a nearly identical climate, one which had been more intensively exploited for thousands of years. I.e the animals had adapted to Norse hunting methods, the best land was already claimed etc.
Furthermore, the Norse were people whose parents, grandparents etc had settled Iceland, a poorer and less welcoming environment. So they had very recent colonization experience not just in their cultural memory, but in some case in the immediate family.
It is rather telling how matter-of-fact and successfully they settled on and adapted to Greenland, a far more hostile place and much further outside their climate coping toolkit than Vinland. Imagine the pilgrims etc from Britain landing in Greenland, how would that have gone? The difference between the Norse climate coping skills and the Europeans were probably similar to the difference in skiing skills between UK soldiers and Scandinavian ones that we enjoy every time there is a NATO maneuver
Military conflict, I would imagine the Viking would be killing each other. In any case, I doubt military conflict killed much in the population even in Europe.
Probably true, outside of certain periods. But I imagine the ability to just move away would limit intercine violence a bit.
Food scarcity was a problem in the early American settlements and it will be until it gets going.
As I said, very different skill groups there. If you come from a place climatically similar to Vinland but slightly worse, with the cultural toolkit to have no starvation issue when settling Greenland, a more similar but richer environment than your origin area should not be a problem
A community of half breeds would adapt much better. see (a) above.
Yeah, no. The reason the agriculturalists had such a massive advantage in population density is that they were better at resource extraction and production. The Inuit had a lot of skills they could have taught the Norse. The agriculturalist Three Sisters complex inland had a lot of thing to add. This is where we could potentially get hybrid societies.
The Newfoundlander natives hit their demographic limit at around 750-1500 people over an area bigger than Iceland. The Norse had bigger towns in similar climates. There is nothing unusual about this, its the normal agriculturalist vs. Hunter-Gatherer story. The twist is that the Agriculturalists have many bonus advantages here.
Also: The population of Newfoundland at the time of Erik the Red was about 750 people. At a normal population distribution, how many were females who could pair up? 1/10th maybe? So 75 women. Erik the Red probably set out with about a 1000 people. Mostly young, I expect. And that is the best point for the natives, population growth will not do them any favors from there on. Unless we are postulating some kind of severe population bottleneck, like a single wrecked ship, the natives, as hunter-gatherers don't have the numbers.
You would need there to be something in Labrador that the Norse couldn't get elsewhere.
Vinland failed not because of Native attacks but because it wasn't economically viable
Free fertile land. That is why I said that the windows were before the investment in settlement of Greenland, and probably just as Greenland were failing. Once they had an effectively infinite supply of lower-grade land in Greenalnd, that ceased to be a draw.