I now have completed the core upgrade successfully by following the upgrade steps 1 – 13 in https://drupal.org/node/570162 using D7.24 and the latest version of the ‘repair’ script as I now call it.
Step 14 was to take a database backup, substituted by a snapshot. Then step 15 was to Upgrade Fields, meaning migrate data from D6 CCK into D7 Fields within core. (There are some notes in Pages on my iMac I need to insert here later.) And that got me a snapshot of a completed Core Upgrade that I can roll back to in a pinch