These are written to replace the date created column, however, if you wish to keep it, simply add the codes from part 2&3 after the date section rather than replacing it.
In include/staff/tickets.inc.php ~ line 140
Modify this
$sortOptions=array('date'=>'ticket.created','ID'=> 'ticketID','pri'=>'priority_urgency','dept'=>'dept _name', 'ass'=>'username', 'from' => 'name', 'subject' => 'subject', 'updated' => 'updated');
to include
'updated' => 'updated'
In include/staff/tickets.inc.php ~ line 333
Change this
<?=$qstr?>" title="Sort By Date <?=$negorder?>">Date
To be
<?=$qstr?>" title="Sort By Response <?=$negorder?>">Response
In include/staff/tickets.inc.php ~ line 377
CHANGE
<?=Format:($row)?>
To this
<? if($row!="0000-00-00 00") echo Format:($row);
else echo "none";?>