This is a difficult question to respond to. I think it would be best to approach the first two specific questions you pose.

First and foremost, Virgil was read throughout the Middle Ages. The fourth Eclogue led many to believe that Virgil was a prescient pagan that knew of the nativity. This interest has handed an extensive manuscript tradition down to us. A dedication of Virgil caused the fourth century author Proba rearranged Virgillian verse to recreate the narrative of the Bible. This text was widely circulated for many years.

Now let us turn to your specific questions.

Regarding your sixth century Welsh prince: Bede was certainly the most well read individual in the British Isles before the age of Alfred the Great. Bede's corpus includes a great many works about a number of topics. For our purposes, however, we need only look to the Ecclesiastical History. Bede dedicated this work to the local petty king. This action suggests that some local rulers had access to monastic learning. Unfortunately, it remains unlikely that a prince in Whales would have had access to a monastic library as extensive as Bede's given his peripheral location.

A 12th century north German duke is another interesting question. To get to the basis of his knowledge of Greco-Roman mythology, I would look into the scholarly output of Alcuin. Alcuin was the central figure in the "Carolingian Renaissance" and lived throughout much of the second half of the eighth century. Alcuin taught many of the great thinkers that followed him and thus, laid the groundwork for the intellectual culture of the proposed 12th century duke.

Regarding his Greco-Roman connections, Alcuin certainly exhibited Ciceronian pretensions. Whether this was acquired through knowledge of Ciceronian texts or through instruction is beyond me. Certainly Charlemagne's historian, Einhard, displays knowledge of Suetonius and other Roman historians. Whether actual Greco-Roman mythology was well known at this time is difficult to say.

The Italians are another difficult question. Especially over the 600 period you're asking about. Mark Musa's notes to his translation of The Inferno suggest that at best, Dante had access to a translation of Homer into Latin, but he likely hadn't read Homer. This is just one moment in the 600 year span you're asking about.

I think the best way to get at the the overall premise of this question would be an exploration of the manuscript history of Ovid's Metamorphoses. Even today, Ovid continues to be the source of much of our knowledge of these stories and the manuscript tradition would at least point to locations where knowledge of Greco-Roman stories were recorded.

Richard Treat Bruère wrote an article titled, "The Manuscript Tradition of Ovid's Metamorphoses." This begins to answer your question, but it only indicates that knowledge of the mythology you're asking about is transmitted through Ovid's verse. You're asking about a huge population of individuals with varying levels of access to these texts. Beyond that, however, I can't really answer your question.