Ok, please can I clarify what I need to do.
1. Start by making my revised .mobi file. Use the existing MobiUnpack and edit the extracted source, fixing the font problem and adding the code for the start postion. Drop the opf onto KP and I have my revised .mobi file.
2. Having installed Python (from where?), run the script, point it at my revised .mobi file, creating a fixed version.
3. Test the fixed version on the device.
4. If it is OK, upload the revised unfixed file to KDP.
And a question. If the fixed file is put on a device where it didn't need to be fixed, e.g. the Touch, does it work OK or did it become broken by being fixed?
Finally, if one uploaded the fixed file to KDP what would happen?