Yes.  You can say either:

In the old AdventureWorks2016 database
In ye olde AdventureWorks2016 database

but I don't think they should be mixed 😉

Without seeing the actual MERGE, can't get into specifics.

But could you try batching it yourself.  For example, if the table being MREGEd has a unique clustering key, then...

I think it's more clear and more flexible to just test each and add up a "not null" count accordingly:

CASE WHEN
    CASE WHEN...

Maybe this?:

SET ANSI_NULLS ON;
SET QUOTED_IDENTIFIER ON;
GO
ALTER Procedure [dbo].[sp_MarkAsLeaver]
    @username varchar(50),
    @LeaverDetails varchar(100),
    @RowsUpdated int output,
   ...

Something like this:

SET ANSI_NULLS ON;
SET QUOTED_IDENTIFIER ON;
GO
CREATE PROCEDURE [dbo].[AppTracker_Milestone_Dates_Browse_SP]
    @UserName VARCHAR(51),
    @DecisionDate DATE,
    @State...

Maybe two separate hashes: (1) all the other columns and (2) the 4K column alone?