Recover email archive spanning 20 years from corrupted Apple Mail files on an iMac

I have a 2011 iMac (2.5 GHz Intel Core i5) running OSX El Capitan, and rely on Apple Mail (v. 9.3, 3124) for email. I am limited to 1 GB by my mail provider, so archive most messages on my local hard drive, in folders within Apple Mail.

In July 2019, I created a new user account on this iMac and migrated to it. However, while most email folders copied properly, some did not. They appear to be corrupted.

I would like to recover those messages. There are tens of thousands of individual emails, dating back to 2001. One of the key .mbox files is 124 MB, while the other is 656 MB. While a full recovery of text and attachments is unlikely, even a partial restoration would have value.

A screen shot of the files resulting from my multiple efforts to recover the files is attached.