I keep having to remind myself of the 'basics'...
I always tell new developers "If it worked before the changes you made and you're certain the changes didn't cause the aberration you're now seeing, then shut down and restart Visual Studio. If you still see problems, reboot the box. If the problems still exist, you're wrong about your changes not causing the aberration".
This applies across the board, and for some reason I keep forgetting the whole of the process when working with WPF/E.
When I was uploading my WPF/E MIX Count-down 'gadget' code, I had some major issues. The header WPF/E was not running, and the article WPF/E was not running, although the sidebar was.
I finally went through my MasterPage code and changed every JS variable name associated with the MIX WPF/E just to make sure it wasn't interfering with the other and then the two 'gadgets' ran fine. As an upshot of that, all but one of my WPF/E pages also worked fine with the count-down timer in place, although the header WPF/E was still absent.
None of that code had changed, and over a month ago I had instituted a macro in my editor to avoid clashes between two different WPF/E modules on a page, so I didn't see how that could be a problem, and no amount of looking at code showed what was wrong.
As I usually do, I shut down IE overnight.
The next day when I opened my site, the header WPF/E object was working. Upon investigation, all the WPF/E objects were working as well.
The bottom line is, I didn't follow my own rule, or I suppose there needs to be a corollary to the rule as it applies to WPF/E: shut down and restart your browser.
Never be afraid to restart VS in the course of debug, never be afraid to restart the box, and apparently never be afraid to restart your browser!