Yes, it is working. Don't try exporting and re-importing to verify formats because the exported header type is IN and the required import header type is I. The real problem however, was a string that had somehow crept into a numeric field. It is a very large file and I overlooked this when checking it visually. The error was deep in the file, yet the import stopped at the outset.

All's well that ends well. The client is smiling once again.