A British America is a different dynamic from US-Canada. An example of that would be a different pattern of migration, leaving the prairies less settled.
I see no reason why that would happen. There's simply too much money to be made off of settlement to allow it.
The problem with the Norse option has is that in the 11th century Scandinavia (and to a large extent Northern Europe) is relatively disease free still - particularly the big killers like smallpox and measles. An early introduction of whooping cough will save some lives in the Americas, but without contact lasting until roughly the 13th century the Americas are still going to be devastated by later disease influx.
Absolutely. The issue with contact from most of the likely candidate is that they're both small and peripheral. Its even worse when you consider the proposed contact times are usually short. For disease spread to occur in a way that isn't going to devastate the Americas you need sustained contact with Afro-Eurasia, and you also need it to be a slow enough contact so the Americas aren't outright conquered.
Diseases change over time as well, so what strain the populace has become used to will not necessarily guarantee a immune Native American population. However, with the introduction of isolated domesticated animals, the Europeans could deal with devastating outbreaks of New World strains of illness that they haven't come in contact with before. Just a thought
