Presumably yes, for the same reason.  The total I/O will be spread across two files rather than one.

SELECT LEFT(first_name, 10) + CASE WHEN LEN(first_name) > 10 THEN '...' ELSE '' END AS first_name 
FROM @t1

I've noticed, and been impressed by, the overall great performance of the ROW_NUMBER() functions.

It's still a falsehood.  The docs say it is the "false_value", but the result is not "false".  ELSE is accurate, because that simply means the WHEN condition was...

Like a COALESCE with 10 arguments,...

The rule for SQL expressions and functions is that if a NULL value goes in, a NULL value comes back, unless you explicitly do something to change that.

IIF violates, in...

Like a COALESCE with 10 arguments, for example....

CLR functions for performing Regular Expression searches aren't SQL either, but they're useful. Where do you draw the line?

IIF is part of T-SQL and is more succinct...

You should just be able to add this to the end of your existing calc:

(DATEDIFF...
- (SELECT COUNT(*) FROM dbo.Holidays H WHERE H.date BETWEEN JobStartDate AND JobEndDate)
AS ...

CLR functions for performing Regular Expression searches aren't SQL either, but they're useful. Where do you draw the line?

IIF is part of T-SQL and is more succinct than CASE,...

I don't see the need for all that, this is much simpler:

SET @ytdfrDate = DATEFROMPARTS(YEAR(@frdate) - 
    CASE WHEN MONTH(@frdate) < 4...

Only with a bad data model.  Parts from the same invoice line could be shipped on different dates.  You don't split...

I don't see the need for all that, this is much simpler:

SET @ytdfrDate = DATEFROMPARTS(YEAR(@frdate) - 
    CASE WHEN MONTH(@frdate) < 4 THEN 1...

I'm more confused than...

CONVERT works on a single value at a time.  Thus, the query should be more like:

and a.myDate BETWEEN CONVERT(date, '20201108') AND CONVERT(date, '20201116')

The strings will have to be converted to...