Your under the illusion that they ask about the sign of his coming meaning his second coming but if you examine the Gospels in every case when Jesus spoke to them of his death they did not understand. So by the timeline of the Olivet discourse they(the disciples) did not understand or believe in a "second coming" and so were asking him about the "sign of thy coming"
The disciples knew Jesus was going away. Let's look at the next 2 verses in Matthew 24 and keep things in context:
Matt 24: 4-5, "And Jesus answered and said to them: “Take heed that no one deceives YOU. 5 For many will come in My name, saying, ‘I am the Christ,’ and will deceive many.
Jesus is speaking to the disciples. He is not speaking to you and me. What you just quoted is straight out of many commentaries who try and say the disciples were confused. They were not. They knew He was going away.
So in Matthew 24:3 they ask what was the sign of your coming as the king,Messiah that is going to rule the world from the Temple you just said would not be standing for him to rule in. And the age they were in when it would end in aspect to the things he said(these things be?),,,The point is though if you examine all three Gospels(after Christ death) none of the Disciples/Apostles undersood he would die,be buried or rise the third day and so it is impossible that they ask him about the sign of his second coming so they had to be asking him about the sign of the only coming they understood as the King,Messiah that rules the world and every nation bows to in the Temple he just said would be destroyed.
The Bible never uses the term "second coming". The entire New Testament speaks of Christ's coming with His kingdom. One coming. It doesn't speak of multiple comings of Christ anywhere. Just one. That's what the disciples were asking about.
So what are you trying to say? I don't get your point. They knew when He came that He would be establishing His kingdom and it would be the end of the Jewish age. That's why they asked the question the way they did.