If you want to limit the delimiters to strictly:dot, comma, underscore and space, in the code below, change:
PATINDEX('%[^A-Z0-9]%'
to
PATINDEX('%[.,_ ]%'

SELECT
    CASE...

Yes, they are the same in SQL Server.

SQL chose not to treat extra decimal positions as en error, instead it implicitly rounds to the stored scale.  I assume...

Fwiw, I usually stick to sp_refreshview for views, and only use the broader module proc when I need to.

Yes, you can combine them into one integer and get them back.  The int value will most likely be negative, I don't know if that matters for you.

The...

If you (almost) always look up on tableB by the MName, then cluster the table on that.  That would prevent a full scan of the table from happening every time...

Isn't this the third iteration of the same essential q?  Why do you keep re-posting the same q??

I suggest just this.  If that still performs poorly, we'll have to look at the view itself and tune it.

DECLARE @StartDate date = '01-01-2018'
,...

SELECT
    EmployeeId, Forename, Surname, DateOfBirth, leaver, EmployeeStartDate, LeftDate
FROM (
    SELECT *, ROW_NUMBER() OVER(PARTITION BY EmployeeId
        ORDER...