In the wee hours of the morning last night I figured out what was tripping me up.  enigma_mime_message::contentHeaders is overriding the same call in Mail_mime.  So I added logic in enigma_mime_message for SMIME signed messages, scraping the SMIME signature out of the openssl output and reusing the addPGPsignature method to inject it in the outgoing email.  I just need to sort out what headers need to be included when signing (vs sending) and it should be working.

What does everyone use for debugging purposes?  I tried using NetBeans/xdebug but it didn't really work for me.  Any other recommendations besides file_put_contents?