In very simplistic terms, I'd probably do something like this:
CREATE TABLE #Dates (StartDate DateTime, EndDate DateTime);

INSERT INTO #Dates (StartDate, EndDate)
VALUES ('20170428 15:00:00', '20170504 10:00:00');

I'm not even going to try and defend my English. I live in England, always have, and English is my only language. I, however, know all to well that I...

Use the OUTPUT statement
UPDATE Table1
SET Count = 0
OUTPUT inserted.* --Or list your columns, i.e. inserted.CustomerName, inserted.Postcode, etc, etc
INTO #Output
FROM Table1 AS T1 JOIN...

I would go with a staging table, and upload the contents on your CSV file(s) to it. SSIS can do this part using a data flow task more than easily.

I think what you're after here is a CROSS APPLY, so you can limit your result that way.

This is untested, however, instead of your INNER JOIN to [CustomerBrokerView]...

We need it in a consumable format, please. Have a look at the link in my signature on how to achieve this.

The screenshot doesn't show where the data is coming from, so all we can do is guess.

If you supply DDL and consumable sample data that shows the...

An Auto incrementing foreign key? What is your goal here?