Many thanks for your detailed response. I did draft a response yesterday but it looks as though I got stuck at the Preview stage and didn't do the Submit. Sorry.
Anyway, I've now been able to research on my system the points that you highlighted and I can confirm the following:
My gtk2-default-theme.rc.sh is in $HOME/.kde/env and is the same as yours and has executable permissions.
My $HOME/.gtkrc-2.0-kde4 is similar to yours but the gtk-fallback-icon-theme is Gnome rather than oxymentary (and the font is DejaVu rather than Droid).
Unfortunately I couldn't find a way to access 'systemsettings'. A normal boot produced a desktop where the only program I could run was Firefox and the keyboard shortcuts to launch a run command or terminal didn't work either. I then tried using the recovery boot but of course X-windows wasn't running and the program aborted "Unable to autolaunch -daemon without a $DISPLAY for X11".
I have been able to set up a new version of KDE Mint on an old hard drive I had and from that I can access my damaged setup but if I try to run System Settings it of course refers to the one in the new installation, not the instance in the old installation. I'm reluctant to put much on to this drive as it's now 12-13 years old so repair of the old installation is still the preferred option, though I can now do proper backups and reinstall I suppose. Just seems a bit drastic for a simple, if very stupid, mistake.
I have also found " How to auto load a script on KDE startup " on https://www.linuxquestions.org/question ... up-177441/
and started to hope again.
Sadly placing the "gtk2-default-theme.rc.sh" in either of the locations mentioned failed to give me a functioning desktop though there was slightly more activity when I used the "rc.local" file - i.e. the Start Menu appeared, but it wouldn't go any further and launch any of the entries. The postings date from 2004, though, so the procedures for KDE 4+ may well be different to those described.
Somehow or other I think I need to find out what actions Autostart actually takes when the script entries are processed and see if I can implement them manually. However "systemsettings" is a binary file and 'whereis autostart' gives me a directory in /usr/share, so i'm not really any further forward.
I don't fancy ploughing through source code for this, given my level of understanding, but don't know of any other possible source for the required information.
If you're able to help at all further then that would be great but I quite understand if this is getting more detailed than you want or have the time to go into.
Your help so far is very much appreciated.