I conclude a false negative (manual consistency check deems healthy) IS worth looking into (not so false after all).
Out of many dozens of test backups that had run on file, only a few failed the back up consistency check. However, as it turns out, the file did have issues revealed with recovery.
After various recovery test efforts I came up with this:
To avoid excessive recovery issues, tune up* first in FMP11. Then inFM15 Recover fully and and review remaining issues in log†. Aside from the easily fixable index issues, I found that if I deleted the layout causing the error, and the table causing another, I got a clean bill of health in final recovery tests.
BTW - I decided to make those changes in FM11 (kill a layout and a table) to give FM15 the best chance at a 'blessed' conversion.
* Tune up: Recovery as compacted with only rebuild indexes and clear caches checked — this gives a usable production file as per an FMI tech
† search for "from old index is -" "Resetting invalid" "Deleted invalid"
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