Hmmm, don't know about having the divergence point as far back as the Roman Empire. Earth in the MU is still quite similar culturally in some aspects to the regular Earth, so I reckon the divergence occured closer to the current era (There's also the fact that MU-Phlox says that Shakespeare's plays were identical in both universes, something I don't think would be possible if MU history had gone askew before then; Shakespeare was very much a product of his era, and a lot of his plays were about historical characters living centuries before his time).
IMHO, the divergence point couldn't have occured further back than around 3/400 years prior to In A Mirror Darkly in order for the culture of the Terran Empire to have that many similarities to its UFP opposite.