Regarding your third question the answer is "yes, broadly speaking". Vikings has made a commendable effort to use what we do know of period combat formations in combination with what they think appeals cinematically to form battles that are with a reasonable margin of error of what they may have been in the 9th century.

The important thing to remember is that, despite the fact that shieldwalls were used back as far as 2500 BC and that we have reasonable documentation of them from the Greeks, Romans and the medieval period, there are still a lot of contentious issues regarding the actual mechanics of combat. Ask a group of people who study hoplite warfare how battles were fought and you'll get three or four different answers.

Regarding 9th century Scandinavian and Anglo-Saxon warfare, the two sides would typically face off each other and either match their frontage with the enemy by thinning their ranks or by using natural obstacles to protect their flanks.

As the two sides advanced, there would be an exchange of missiles (arrows, javelins and sometimes axes, with the Vikings probably being responsible for a large portion of the arrows), followed by the two shieldwalls coming together. The idea was to cause enough damage to the enemy that a rout began, and the easiest and most cost effective way was to break their shieldwall, since they would then essentially be able to attack the two newly formed formations from the flanks and rear. The skirmishers from the opening stages of the battle probably kept a watch for lone fighters or small groups of warriors sneaking around the flanks of their formation.

Because the easiest way to win was to break the enemy shieldwall, a wedge shaped formation called the "boar's head/snout" was used by both sides. Its use probably depended on factors such as proportions of armoured warriors on both sides, terrain, size of the relative forces, etc, but nothing can be said for certain, because we have very few detailed records.

Finally, I'd advise you to ignore every battle you see on TV or in movies. It's very, very, very rare that they even get the most basic elements right. Vikings is something of an exception to this rule but, even then, isn't perfect.

The short answer is we don't really know. We don't really know much about the Vikings at all, at least relatively speaking. There are examples in the sagas of sexual activity like that but it typically involves slaves rather than wives. It also should be said that while the Norse world was definitely a masculine world, it was not so rigid a patriarchy as one might think. Women could reach significant prominence and we're typically able to exercise reasonable agency. There's examples like uum the deep minded, or freydis who convinced her husband to kill his own brother, and then murdered the brothers wife in cold blood, or the oseburg ship burial which is the grave of a woman. There's also an example of a wife in iceland, after converting to Christianity ceasing all sexual relations with her husband in the sagas. The show is usually pretty good at basing what they portray on something found in the sagas, but they typically choose the most extreme thing, I.e. the blood eagle. So I'm sure there is something that documents this but my experience says practices weren't common. I've personally never seen an example of the large orgies in a religious context in a source, but that by no means means there isn't one, it is likely a Christian source however and must be taken with a grain of salt.

I can partially address the third question by saying that everything I've seen indicates that battles were fought that way, but a historian who specializes in combat would likely have a better answer. And I don't have a great understanding of the Anglo-Saxon's so you're on your own there too.

Regarding your first question, no it's unlikely that Rome was ever forgotten. Now, we have no idea what the peasants (which is the vast, vast majority of the population) knew. Maybe they did think that. I tend to think peasantry were probably slightly more informed than people give them credit for and that projecting widespread ignorance on the past is a form of presentism but my opinion is completely irrelevant. But there is no reason to assume they didn't know about the Romans. But the aristocracy absolutely knew.

First, Rome and the Romans are all over the New Testament (render onto Caesar what is Caesar's). They crucified Jesus! This is the sort of thing people would be very aware of. Even if they couldn't read the Bible themselves the Gospels would absolutely be read out to them. They are the most crucial part of the Christian faith. Beyond the New Testament there are also many saints who are of Roman origin or who were martyred by Romans. Constantine, himself, is a major figure in the Church. These are all figures people would be aware of.

Second, as you say the clergy would obviously be aware. And the clergy in Anglo-Saxon England wouldn't just be well traveled but were occasionally from places still ruled by Rome. Theodore of Tarsus became the Archbishop of Canterbury in 668 and he was from the still existent Eastern Roman Empire. He was not alone. There were monks from North Africa too. But what it seems that the show is missing as that these people aren't just in their own world. They would interact with people everyday. Canterbury even then was an important administrative position as well as religious one. The Church had a huge role to play in running the Anglo-Saxon kingdoms. Theodore started a school at Canterbury, which instructed in Latin and Greek and taught about the lives of Saints. Point being these clergy wouldn't just horde this knowledge for themselves. They educated other clergy who would disperse into communities. They interacted freely with the nobility. They educated the sons of nobles. They shared their knowledge.

Third, Anglo-Saxon England was not as remote as the writers of this show seem to be envisioning it. Anglo-Saxon kings were in contact with mainland rulers including Charlemagne and his descendants. Charlemagne, who took the title Holy Roman Emperor, was obviously very aware of the Roman legacy and wished to echo it. Beyond royalty there was a lot of cross-culture. Northumbria was a respected center of learning. Charlemagne brought Alcuin of York from this area to be a scholar at his court. There was far too much cross-culture from the Anglo-Saxon England and the continent for there to be no knowledge of the Romans, who were still known and revered there.

Fourth, this seems to be based in the concept of the Dark Ages, which is largely discredited (although there is pushback against it being discredited in some circles). It is true that the Anglo-Saxons couldn't replicate what the Romans did. They couldn't come close. But the concepts of a lot of that architecture wouldn't have necessarily been so alien to them that they assumed it was magical beings. It assumes that they wouldn't even understand the concept of roads and walls and villas, which we have no reason to believe.

Fifth, literate people (which would have been a fragment but not non-existent) would have read the works of Romans like Ovid, Virgil and Boethius. Not to mention Augustine. And City of God is heavily focused on Rome.

While I don't know I think some of this must be based on an Anglo-Saxon poem The Ruin. Some of the poem has been lost. There is one reference to the ruins of a Roman site (probably Bath) being the work of giants. There is absolutely no reason to take that literally (Anglo-Saxons were capable of figurative thinking). And there is a lot of reasons not to. The poem imagines warriors using those halls. It is clear that the poet knew it was inhabited by human beings not actual giants. A lot of the work imagines that what went on in the halls was similar to how contemporary men behaved not Romans but you can't expect everyday life of a Roman to be well known. Point is they knew it was the domain of men not giants despite a literary touch in one line.

Generally, we know that the upper classes had to be aware of Roman people because they would have interacted with clergy, who we know were. And they had more contact with continental Europe than many imagine. The dark ages weren't as dark as the creators of the show like to imagine, I think.

I wish I knew the answer to your second question. Because that seems like knowledge worth having! Your third question was covered excellently above. It seems commendable that they feature the shield wall, which is often ignored in television and movies of the time (the Last Kingdom does this as well) but was the major feature of a battle at this time.