After many hours trying to debug why the chart would not appear in the v2_dev instance, I took Greg's advice and created a copy of the v2_prod instance, aimed it at the v2_dev backend db, and then gradually added in the modifications I had added in to the v2_dev instance until something broke. It turns out that in the /search.php page I had included a php echo statement.
<?php echo '<p>/search.php.136 <br />' . $_SESSION['finalQuery'] . "</p>" ?>
That line was the problem when that file was invoked by json as an ajax call. Sending that output somehow caused the call to fail - I suspect something to do with headers not being sent first, but I haven't figured it out for sure.
Anyway, I now have v3_dev which has all the new interface elements Simon wanted (with the exception of minimum pardons), and connects to the current data (which is in the dev db).
The counts in the charting seem to be correct now too. The only thing not working properly in the charting is the stacking. See next post.