...That leaves only the spiritual, invisible to living people, body which the book says Jesus will come back in...
I believe the "spiritual body" of the resurrection (1 Corinthians 15:44) is that which is immortal flesh, like the resurrected body Jesus has (Luke 24:39). We are all now in "natural" (1 Corinthians 15:44) mortal bodies that descended by the natural process from Adam's flesh after it was made mortal. At the resurrection we will receive bodies like the glorious body Jesus now has (Philippians 3:21), not by a natural process but by the same Spirit that raised Jesus: "If the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you" (Romans 8:11).
What would be the reason for the resurrection of our bodies if only spirit was good? Dont we already have spirits? (1 Thessalonians 5:23)
If Jesus fleshly body was not raised, why is the tomb empty (John 20:1-9), and why does Jesus still have the wounds of the crucifixion? (Zechariah 13:6)
"Reach hither thy finger, and behold my hands; and reach hither thy hand, and thrust it into my side: and be not faithless, but believing" (John 20:27).
"Behold my hands and my feet, that it is I myself: handle me, and see; for
a spirit hath not flesh and bones, as ye see me have" (Luke 24:39).
...the spiritual, invisible to living people, body...
Again, I believe Jesus' 2nd coming must be physical because Jesus must come back from heaven (Acts 3:20-21) just as physically as he left: "This same Jesus, which is taken up from you into heaven, shall so come in like manner
as ye [the living] have seen him go into heaven" (Acts 1:11). He physically went from the Mount of Olives to the clouds to heaven. He will physically return from heaven to the clouds to the Mount of Olives: "And his feet shall stand in that day upon the mount of Olives" (Zechariah 14:3-4); "And one shall say unto him, What are these wounds in thine hands?" (Zechariah 13:6)
I believe Jesus' 2nd coming must be physical because it must be seen by every eye of every tribe and kindred on earth, causing them great grief (Matthew 24:30 and Revelation 1:7).
...Where would Jesus return on earth so that every living eye would see Him, yet no one would say "there it is'"? Luke 17 : 21...
Note that in Luke 17:20-21, Jesus was replying to the Pharisees, who expected Christ to conquer the world physically at his 1st coming. He was trying to get them to realize that they had to be conquered on the inside first -- as he had told them previously: all they cared about was externals (Luke 11:39).
Notice that right after Luke 17:20-21, Jesus goes on to explain to his disciples that he had to die first, but that after that he would be revealed to destroy all his enemies (Luke 17:22-30). This is confirmed in other passages, which also foretell the day when the kingdom would come with observation (Revelation 1:7, Matthew 24:30).