isnull te devolvería true o false dependiendo de si el valor es nulo o no.
Prueba la consulta con Nz en Access 2002 directamente a ver si funciona, si funciona a lo mejor no lo soporta el Microsoft.Jet.OLEDB.4.0
en caso de que no te funcione podrías probar esto
Código:
SELECT FORMAT(FechaFactura,'dd/mm/yyyy') AS Fecha,
NumFactura AS Factura, NumCliente AS Cliente, Importe AS Total
FROM Facturas
WHERE IVA is NULL AND Importe IS NOT NULL
.....
UNION
SELECT FORMAT(FechaFactura,'dd/mm/yyyy') AS Fecha,
NumFactura AS Factura, NumCliente AS Cliente, IVA AS Total
FROM Facturas
WHERE IVA is NOT NULL AND Importe IS NULL
....
UNION
SELECT FORMAT(FechaFactura,'dd/mm/yyyy') AS Fecha,
NumFactura AS Factura, NumCliente AS Cliente, 0 AS Total
FROM Facturas
WHERE IVA is NULL AND Importe IS NULL
....
UNION
SELECT FORMAT(FechaFactura,'dd/mm/yyyy') AS Fecha,
NumFactura AS Factura, NumCliente AS Cliente, Importe + IVA AS Total
FROM Facturas
WHERE IVA is NOT NULL AND Importe IS NOT NULL
....
es un poco más lioso pero debería funcionar