If the table resides in a separate db, then you reference the table you will need to include the db name, either...

No.  It's random which you get if there are multiple matches.  If you want a specific match, write your own code to do that.  For example:

Separate new filegroup(s) are fine, you don't really need a separate db, and you don't want to deal with the three-part-naming issues that a separate db would require.

Effectively yes.  If you're updating a single row in a non-columnstore table, then the main row (from the clus index or from the heap) must be read in order to...

Nah, ISNULL() in a SET is fine.  It's only in WHERE and join-related clauses that it makes a difference.

If you're going to use a name, explicitly specify it as a job name:

exec msdb.dbo.sp_stop_job @job_name = @jobname ;

The column "Totalspace" doesn't exist any more: the columns in the inner query are "minval" and "maxval".

So, something like:

select
  databasename, minval, maxval,
  ...