The implied opponents in 2 Peter denied the return of Jesus (1:16, 2:1-3, 14, 18). There are several reasons for this, but primarily it was because the first generation of believers were old or dead. Peter himself is about to die, Paul will die about the same time. Yet Jesus has not returned – why is this?

I think that it is more like that the phrase “cleverly devised myth” implies that they opponents claimed that the (Jewish) apostles over-interpreted the words of Jesus because of there apocalyptic world view. As the church became increasingly Gentile, it became more rational. The second and third generation Gentile believers were not reading Daniel and 1 Enoch, they were reading Stoic and Epicurean philosophy. As a result the “apocalyptic” aspect of early Christianity was muted. These false-teachers deny the return of Jesus because they do not share the apocalyptic assumptions of Paul and Peter! (This suggestion has the advantage of explaining the missing text from Jude, especially the citation of 1 Enoch which concerns the apocalyptic return of the Lord. Peter avoided them since they would cause more trouble from his opponents.)
The opponents also denied a future judgment as well as the return of Jesus. The coming of Messiah is bound up with the idea of a judgment on the nations in Jewish apocalyptic. When Messiah comes, he will judge the nations and punish those who are not considered “righteous.” In Matt 25: 31-46, for example, when Jesus returns he will punish the nations that mistreated his children. If the Messiah is not coming back, then he is also not going to judge people for their present behavior (2:19).
Peter’s strategy for countering his opponents is interesting especially since we now live some 2000 years after Jesus. It is fairly easy to mock the idea of a “return of Jesus” given that he has been away for quite some time, and some of his followers keep failing at predictions of the day and hour. Rather than point to so-called fulfilled prophecies or trends in society which “prove” Jesus is coming very soon, Peter argues first that God keeps his promises, even if there is a long time between promise and fulfillment. Second, if there is a delay, that delay is a reflection of God’s mercy and his hope that those facing judgment will repent. I think this is the point of 3:8 (“a day is like a thousand years”) is to point out that God often gives a long time for repentance.
Does this sort of “strategy” work today? How does a Christian firmly hold to the return of Jesus while separating from the more embarrassing examples of recent years?
cloud declaring Jesus to be his son evokes both the theophany at Mount Sinai and Psalm 2, an important messianic Psalms.
crucified (Acts of Peter). There is nothing historical in these stories since the point of Jesus’ prediction was only to tell Peter that his faith was not weak and that he would persevere all the way to the end.
culture. If you really cannot see well, you find yourself in difficult, embarrassing, or potentially dangerous situations. It is one thing to not be able to read a menu in a restaurant, for example, and quite another to not be able to tell one person from another. Worse yet, if you cannot see well enough to read street signs, driving a car becomes very dangerous.
said you would do, etc. Like blindness, forgetfulness can lead to embarrassment (who are you again?) but also to potential danger. Think of people who have serious problems remembering who they are, such as an amnesiac or a person suffering from Alzheimer syndrome. It might be dangerous for a person to be on their own because they have forgotten critically important information that will keep them out of danger.
Pursuit of virtue must be a strenuous effort on the part of the believer. “Make every effort” implies deliberate action. Someone might claim to be growing in godliness, but if there is no deliberate activity then the claim is empty. Imagine someone who claims to be trying to lose some weight, but they are not dieting or exercising. They are not really making “every effort” to lose weight! This is a bit like a “good faith effort” in modern English, but perhaps stronger. It means that the person really does make an honest effort to pursue virtue and godliness.