EUREKA!

Thank you raven: you have saved my project! I still have no idea why the Main Camera was being such a pain in the arse, mind you, so some day I'll probably still need to figure this out, but using the Face Camera works just fine!

thank you thank you thank you

If anyone has any insight as to why the Main Camera woud alter the angle of the render depending on it's Focal Length (and in an entirely different way than the way the image is affected in the Preview window) I'm still all ears.

But thank you for the suggestion, raven! A more than adequate "band aid".