not IS NULL; IsNull()
You don't want the SQL predicate (I think that's the word), you want the vba function.
For your first query, try:
IIf(IsNull([QUERY_A]![QTY1]), 0, (IIf(IsNull([QUERY_B]![QTY2]), 0, ([QUERY_B]![QTY2]*[QUERY_A]![QTY1]))))
hth
mcalex