Before I did all that, I decided to look closely at the code once again. The previous developer used the Code Snippets plugin and wrote code to augment the output of the WooCommerce Export CSV Order plugin. I use the functionality in a handful of sites I manage, but the Code Snippets requires Points & Rewards plugin, which I wasn’t using on this site.
One thing I have not been doing all this time was watching the Checkout process through the error logs. Today I decided to do that. When I saw a line of code that was causing the Internal Server Error, I looked for the Instance Call in the Code Snippets code. When I found it, I commented it out and tried again. This time the order went through in test mode to the final checkout page “order complete” and it generated the file I wanted all along. Now I have the results I require.
Sorry for the inconvenience, but you can close this ticket.