Some answers to your questions:
1. Neuraminidase appears to be important in the release of virus from
infected cells, but the precise function in this context is unclear. It
is known that influenza viruses which lack NA form particles containing
neuraminic acid, which forms large aggregates and result in lower virus
yields.
2. The specificity of the viral NA is terminal scialic acid residues
adjacent to galactose of galactosamine. These biochemical moieties also
are involved in binding of virus to target cells via the hemagglutinin.
If virus is not internalized following binding of virus, the NA can
cleave the receptor and elute the virus from the cell.
3. Influenza remains localized to upper respiratory tract and nevers
establishes viremia.
Jay Mone'
Millersville University
Millersville, PA, USA
