Thank you for the information jondkauf, sadly neither of these pages (as far as I can see) explain what changed between 12.1.6 MP7 to 12.1.6 MP8, which caused the MP8 install to fail on a machine that has Secure Boot enabled, while MP7 did/does not.
According to the document you referenced: http://www.symantec.com/docs/TECH235458 SEP 12.1 RU6 MP5 and later have "Basic Compatibility", I would take that to mean that the installers should work the same as well, that said, it appears the MP7 installer has WHQL signed drivers, whereas I assume MP8 installer does not...that is my issue.
At this point I'm not interested in Device Guard and/or Credential Guard, their relatively new features, but Secure Boot is a different beast, and it has been around and functioning just fine with SEP for years...until now. The more I get the run around on my questions, the more I firmly believe this was a conscious choice on Symantec's part to force their customers from 12.1.x to 14.x.
Enuff said, unless someone can give me some technical reasons why MP7 worked, and MP8 doesn't, I'll save my words until our next contract renewal.