PROBLEM SOLVED Khai, Actually I had both KT and SU installed in non-native directories. Since nothing that was suggested worked, I reinstalled KT INTO the default directory, reinstalled the SU plug in (it asked me where the new KT installation was) and I tried rendering once again. The export process in SU would start and finish without any messages. KT wouldn't start automatically or load the export. BUT when I open KT manually and open the created xml file, the export would load and would render as expected, using all the light sources specified in SU. In contrast, prior to this fix, the manually opened xml files were just blanks. I suspect the solution was the reinstallation of KT. Not sure if the default directory helped. Either way, thanks to all for helping out!

How about giving one set a distinctive texture in addition to colour?

Chris, Thank you for a detailed solution and explanation of the way Sketchup perceives scenes. I tried toggling the hidden attributes but the model would keep on reverting to earlier versions. I was able to solve the problem by Select All > Make Component > Copy of my 'healthy' frame and opening a new file and pasting there. The scenes did not copy into the new file, and starting with a blank Scene list I was able to make the animation. Thank you for your help, it led me to some realizations about Sketchup!