There are multiple verses in the Gospels where Jesus says the second coming will occur within a generation of his death, which is forty years. He said some of his followers would live to see the second coming. He referred to Daniel 7:13 where "the Son of man came with the clouds of heaven...". Matthew 16:28.
The disciples / Apostles will see the Second Coming from where they are in Heaven.
As for your first claim in bold, that's not exactly what Jesus said. Jesus was speaking about the end times when he said, "this generation" (i.e. the generation in the end times). Author Hal Lindsay made that very point in his 70's book, "The Late Great Planet Earth."
Also,
Skeptic: "At Matthew 16:28, Jesus tells his disciples: “There be some standing here, which shall not taste of death, till they see the Son of man coming in his kingdom.” The people who were standing there all died eventually, and they never saw Jesus return to establish a kingdom."
Response: Mark 9:1 notes, "Until they have seen the kingdom of God come with power;"
First, what is the Kingdom of God? Romans 14:17 answers that: "For the kingdom of God is not a matter of eating and drinking, but of righteousness, peace and joy in the Holy Spirit."
Furthermore, in Luke chapter 11:14, Jesus drove out a demon that had left a man mute. The mute was healed. Then, in Luke 11:20, Jesus said, “But if I drive out demons by the finger of God, THEN THE KINGDOM OF GOD HAS COME TO YOU.”
The Kingdom of God is also Jesus being seen in a Kingdom appearance (more on that later).
When did it come with power? In Acts 1:8: “But you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth.”
So, the power would come by virtue of the Holy Spirit. When did that happen?
It (the power) happened in Acts chapter two, verses 1-4: "When the day of Pentecost came, they were all together in one place. Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. They saw what seemed to be tongues of fire that separated and came to rest on each of them. All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them."
After that, Act 5:12 notes the miracles and power of the disciples: "And by the hands of the apostles were many signs and wonders wrought among the people."
But when did some of the disciples see Jesus coming in his kingdom?
"It is that Jesus’ Transfiguration occurs next in the synoptists’ accounts (Mt 17.1-8; Mk 9.2-8; Lk 9.28-36a). Jesus took Peter, James, and John “up a high mountain by themselves. And he was transfigured before them, and his face shone like the sun, and his clothes became dazzling white. Suddenly there appeared to them Moses and Elijah, talking with him” (Mt 17.1-3). From a literary perspective, it seems pretty obvious that all three synoptists intended for their readers to understand that Jesus referred to his upcoming Transfiguration when he said some disciples would see him coming in his kingdom before they die."
Did Jesus’ Disciples See “the Son of Man Coming in his Kingdom”?