It looks as if some of the problems may not be “my fault” after all!
Camtasia has started to revert automatically to its internal webcam as soon as I start my recordings, in spite of the settings being correct and the webcam input showing the correct webcam image (via Camtasia) before I press the recording button. Curiously this is only happening in the lecture room, and not as yet in my office. The last time I had this type of mysterious effect it was related to mains power vs battery, but I don’t think that is the issue this time, as I am (usually) very careful to make sure that I am on mains power.
The problems might be related to some internal settings. Or maybe a conflict between Camtasia and the Logitech webcam driver. Yet I successfully recorded my “follow my face test” video last week.
More testing is in order in order to reproduce this fault under more controlled conditions. Perhaps my laptop just needs a restart!
Until this is resolved, my published screencasts will not include PiP video of me from the webcam.
See comments below for my “solution”!