So considering all of this, is a 10-20% population drop and a reduction in population growth in the areas neighbouring Norse territories realistic? At least until Smallpox arrives, possibly in the 14th century?
Maybe, yeah. I'd say the biggest effect would be on child mortality rates though; Flu and and Pertusis (Whooping Cough) are hardest on the young. Typhus and Typhoid may actually not spread that far from Norse communities, Typhus is spread by fleas and lice, so its mostly going to be hitting agricultural groups, and Typhoid spreads through fecal-oral transmission, so that's mostly going to be relegated to Norse communities, at least for a while. Typhus is probably the only thing that causes many deaths in adult populations outside Vinland I'd say. Child mortality is probably made up for the usual way populations deal with it: having more children.