After more research and experiments let me try to answer my own questions and perhaps you will be kind enough to tell me if I'm getting this right.
The InDesign mobi file contains a mobi7 folder with the old format files. A mobi8 folder with the KF8 format files and a kindlegensrc.zip file.
I can unpack it all, and go to work fixing the problems with the contents of the mobi8 folder. I can change the styles for example.
I can then take the opf file from the mobi8 folder and drop it onto the Previewer, which is equivalent to running Kindlegen from a command line. I can take the resulting compiled mobi file and put it into Calibre. I can delete all the files from the previous unpacking. I can then unpack the new file and by magic I see that I again have everything I had before, the mobi7 folder is back. So the answer is that it is Kindlegen which will automatically read a KF8 set of files, including styles etc, and it creates not only the KF8 but also the KF6 (mobi7) version, all at once in the new mobi file.
In fact I realise that the InDesign Plugin itself uses Kindlegen.
Anyway, all I have to do with this workflow is run the plugin, load the result, unpack it, keep only the mobi8 folder, edit the files as needed, compile using Previewer and submit the result to KDP.
There are two problems with the plugin. One is to do with setting the start position for goto beginning. I have not quite cracked that one yet. The far more serious one is that InDesign uses points to specify font sizes, which is perfectly natural. But the plugin fails to convert these to, for example, ems. The result is very small fonts and the user can't make them large enough for a comfortable read, except by going to the 7th size when they suddenly become far too large. Using ems in the styles solves this.
So all this means I don't need a mobi-pack plugin after all.
Hope I'm not boring you with all this.
Last edited by Tugger; 09-17-2012 at 08:08 AM.