Regarding Marcus Atilius Regulus

[SigniferOne]

Tzetzes may have made up the 'torture' part himself

Not at all. Considering it being mentioned in other sources, it is far more likely that Diodorus, immersed in the cultural atmosphere of his time, spoke about it himself.

Significantly, none of the historians writing about the first Punic war even mentions the 'legendary Regulus' story

This isn't true. Valerius Maximus mentions the incident:

http://books.google.com/books?id=5imDC6 ... =3#PPA6,M1

Notably, the specificity and certainty with which he recounts the story strongly brings to mind an impression of some historical account behind the work. Throughout his work Maximus never quotes poets as accepted historical fact, so there must clearly be historians behind it.

Horace is not 'stating' fact/history, but rather waxing lyrical about supposed Roman ( and Augustus's) virtues....

And yet waxing lyrical has never meant manufacturing stories out of thin air. Even your example about Horatius goes straight to this, because here was a man and a story that everyone believe existed. Just imagine how here would be a well-known consul who won one 1st Punic War battle and lost another one, yet here would come Horace and manufacture a completely fake appendage story about some sort of return and supposed heroics. There's not a single instance of such an action in all of Latin poetry, and Horace's mention indicates that the story was current and commonly accepted to have credence. I'm not saying that the story was true (just as with Horatius), only that in 1st century BC everybody believed it.