Working Days DateDiff function
You have a physical table that has been established for the purpose of managing calendar dates. This is great, but with just a little more work it can be extended to do so much more.

Consider adding a column "DayType", with values like:

0 -> Normal
1 -> Weekend
2 -> Holiday

You could add this easily... right?

Then, you can maintain it with:

update FiscalCalendars
set DayType = case
                  when Holiday is not null then 2
                  when FiscalDayOfWeek in (1, 7) then 1
                  else 0
              end


Now your query becomes quite simple:

select count (*)
from FiscalCalendars
where CalendarDate between @fromDate and @toDate
  and DateType = 0


Index the column with (DateType, CalendarDate)

Maintaining this static data in a function would be a poor use of computational power, and make indexing harder.

Your function would thus become:

create function dbo.WorkWeekDateDiff(@fromDate date, @toDate date)
returns int
as
begin

    /* returns the number of workdays (non-holiday week days) between two dates. */
    declare @result int;

    select @result = (select count(*)
                      from dwd.FiscalCalendars
                      where DateType = 0
                        and CalendarDate between @fromDate and @toDate
    );

    return @result;

end