If you don't use a date in tabular model, you can't mark the column as a date, so you can't use a lot of the built in time intelligence functions......

A cursor for that is all kinds of overkill. How about a simple not exists subquery?

SELECT *
FROM Employee e
WHERE NOT EXISTS (
                    SELECT s.EmployeeID

(Wait, so you can update directly?)

Found this: https://docs.microsoft.com/en-us/sql/relational-databases/security/encryption/encrypt-a-column-of-data?view=sql-server-2017

Wouldn't you have to run this on each column?
UPDATE HumanResources.Employee SET EncryptedNationalIDNumber = EncryptByKey(Key_GUID('SSN_Key_01'), NationalIDNumber); 

You mean by querying the sys.all_columns table and using dynamic SQL to automate it?

Also, how do you fix a "slow database" when you don't know exactly what is slow? Do you have benchmarks? A list of the most used stored procedures?

Of course, if you want to recover the records you deleted, and you're in Simple Recovery, you're out of luck. You'd have to restore an old copy of the database...

???

Recovery models don't have anything to do with what SQL works in a database. That's controlled by the Compatibility Level the database is set to.

A data warehouse is just another database with a star schema instead of a standard 3NF schema.

You're looking for something called Market Basket Analysis. 
This article might be helpful

We can't see your data. How are we supposed to advise you?
PowerBI is sort of cool, but the fact that you can't write back to sources BLOWS.

use SSIS instead?

DECLARE @start_date date
DECLARE @end_date date
SET @start_date = DATEADD(YEAR, -1, GETDATE())
SET @end_date = GETDATE()

Instead of the above value for @start_date, you could do something...

Should be in the ExecutionLog
https://docs.microsoft.com/en-us/sql/reporting-services/report-server/reporting-services-log-files-and-sources?view=sql-server-2017

Here's a really helpful article I found...
https://www.sqlchick.com/entries/2011/2/6/querying-the-report-server-execution-log.html

Sue's right... I think your question is backwards... SSRS reports connect to databases...

Difference between what? The SSI1, SSI2, and SSI3 columns?
If they're dates, use DATEDIFF(), otherwise just subtract.