You're going to need to query the "latest date" separately from the main query.
Something like one of the following...

DECLARE @begdate DATE = (SELECT DATEADD(MONTH, -12,...

In addition to what others have said, if you want the columns marked as 'NULL' to actually have null values rather than text columns with the word "NULL" you'll need...

I'm not sure what you're looking for... Error codes do get generated for specific AG events.
For example :
1480 = role change
35264 = data movement suspended
35265...

Looks like you're trying to over complicate things...

DECLARE @month INT = 12;

SELECT CONVERT(CHAR(7), DATEADD(MONTH, 2, DATEFROMPARTS(YEAR(GETDATE()), @month, 1)), 21);

These should get you pretty close...

USE CodeTest;
GO

IF OBJECT_ID('tempdb..#President') IS NOT NULL
    DROP TABLE #President;
CREATE TABLE #President (
    PresidentID INT IDENTITY (1, 1)...