But opera seems to be dodging the KDE panel? Not what I'd have expected, if matchbox is still running. So I assume kwm supplanted matchbox. Do you know if this is correct?

What I'm thinking is, if this really is matchbox understanding that a KDE panel should be avoided by non-full-screen apps, then any window can set the appropriate wm hints, and receive the same treatment. Like a custom input-window, or a media-player bar, or ...
Otherwise, if this is kwm's doing, then it's as expected: such things require running your own window manager, which we all know is possible, but a lot of fooling around.