I'd start with a high level summary of all the things in tempDB before looking for specific sessions:
USE tempdb;
SELECT SUM(unallocated_extent_page_count) AS FreePages,
  CAST(SUM(unallocated_extent_page_count)/128.0 AS decimal(9,2))...

Does the ID column of Table1 have unique values?  If not, that could be a potential problem since you join Table1 to itself in the subquery.  If it is unique,...

You could try doing a CROSS APPLY such as in this article to operate on the data in a set based fassion:
http://www.sqlservercentral.com/articles/CROSS+APPLY+VALUES+UNPIVOT/91234/
Then you can do aggregates...

Again, I suppose the real issue is how much free space do you need on the disk, not how much unused space do you have in the database.  Have you...

To verify what files are used by a specific database and how much of them are used, you can run this script in the database:
SELECT DB_NAME() AS...

This may be a silly question, but is there a reason you can't schedule the SQL Agent job in SQL Agent?

Was there a reason you couldn't upgrade the development server to 2017?  Typically you'd want to upgrade the development server before upgrading the production server so the system could be...