@niluje, thank you very much for this vast amount of information. It will probably take me a number of days to work through all suggestions.
Concerning the clear screen in the first script - it's actually the first line which doesn't seem to work. I start the script manually from the KSM user script menu, so I still see part of the script files list from the KSM sub-menu kind of "shining through" (but as horizontal lines, not ghosting) which should be completely cleared before the "animation" starts.
EDIT:
Firmware is at 4.20.14622.
I should already have a much more recent version of fbgrab in one of the many subfolders in .adds (whenever I search for a new version of fbink I also always get the latest fbgrab and strace - you never know ...
)