Darn... really thought I had it after trying two more things:
Thing 1:
I was wondering if the field PK_DesignObj_CurrentScreen perhaps referred to some number other than the screen# (which I have confirmed as 15, but which doesn't fire the command). So I did some searching on the wiki and discovered that there is a table Screen_DesignObj in pluto_main which has the field PK_Screen_DesignObj! Well, that sounded a bit right so I checked the associated records with screen #15 as follows:
mysql> select * from Screen_DesignObj where FK_Screen=15;
+---------------------+-----------+--------------+-------+---------+-------------------+---------+---------+---------+--------+-----------+----------+------------+---------------------+--------------+
| PK_Screen_DesignObj | FK_Screen | FK_DesignObj | FK_UI | FK_Skin | FK_DeviceTemplate | AsPopup | Popup_X | Popup_Y | psc_id | psc_batch | psc_user | psc_frozen | psc_mod | psc_restrict |
+---------------------+-----------+--------------+-------+---------+-------------------+---------+---------+---------+--------+-----------+----------+------------+---------------------+--------------+
| 19 | 15 | 1405 | NULL | NULL | NULL | 0 | NULL | NULL | 19 | 318 | 33129 | 0 | 0000-00-00 00:00:00 | NULL |
| 180 | 15 | 4541 | NULL | 6 | NULL | 0 | NULL | NULL | 180 | 324 | 33129 | 0 | 0000-00-00 00:00:00 | NULL |
+---------------------+-----------+--------------+-------+---------+-------------------+---------+---------+---------+--------+-----------+----------+------------+---------------------+--------------+
And saw that there were two records that had the foreign key as 15. Specifically, PK_Screen_DesignObj 19 and 180. So I tried setting the "PK_DesignObj_CurrentScreen" parameter in the "Go Back" command to each of these individually, quick reload router, run outside to dance in front of camera, come back and count to 30 and .... darn... screen stays there. Just to confirm that the quick reload was changing the event setting I even then deleted the value completely from this parameter and redid the test and, yes, the screen disappears neatly as expected. Grrrr.
Thing 2:
Then I noticed that the initial "Goto Screen" command I used to bring up the camera has an "id" field at the beginning. When looking up the command in the Wizard->DCE->Commands section, the description against that command says that it is used to assign an ID to the screen you're bringing up, s.t. it can then be passed to the "Kill Screen" command. Yay, so all I have to do is set this to some arbitrary number, then call the "Kill Screen" command with the same number rather than the "Go Back" command. Only..... I can't find any "Kill Screen" command. Does that no longer exist? I tried, in any case, assigning an ID and then using that in the good old "PK_DesignObj_CurrentScreen" parameter above but no dice.
hmm...
Thing 3:
OK, haven't really tried this yet but this is where I am- I was thinking that I could tweak my logic a bit and, instead of using the "interruption" parameter to prevent the "Goto Screen" command firing during media and web-browsing I could rather try to put this logic into the event criteria themselves. That actually makes a bit more sense... then the whole sequence doesn't start if media or web-browsing is going on. However, I couldn't quite figure out how to check for either of these two things in the criteria section. Any advice?