While it's true that in Acts 9 Paul alone saw Jesus, is there a verse which says Jesus' appearance at the rapture will be unseen by anyone but believers?
...He taught that he was a pattern to believers which should come after him...
Note again that while Paul did say he is a pattern to unbelievers for how longsuffering Jesus can be for the repentance of even the greatest of sinners (1 Timothy 1:15-16), he didn't say that all those who repent will have a heavenly vision of Jesus like he did.
...the Lord "appeared" SUDDENLY to Paul, without warning...
Note again that what came suddenly was the light from heaven (Acts 9:3, 22:6). If we equate this light with Paul's vision of Jesus, then the vision wasn't secret, for those with Paul saw the light.
I don't believe eschatology itself can be heretical because a heretic can't be saved (Galatians 5:20-21), and we aren't saved by our eschatology (Romans 10:9-10).
...scripture does not rule out the possibility that Jesus could return in secret or multiple times...
I believe it does, as we are told how Jesus' 2nd coming/appearing (Hebrews 9:28) will be: just like his ascension in reverse (Acts 1:11), and after the tribulation (Matthew 24:29-30).
...The verses QUOTED by you do NOT refer to the gathering of the body of Christ...
Note that at the 2nd coming some of us elect will still be "alive and remain" (1 Thessalonians 4:15) on the earth: "And then shall he send his angels, and shall
gather together his elect (eklektos) from the four winds, from the uttermost part of the earth to the uttermost part of heaven" (Mark 13:27).
"Elect (eklektos) according to the foreknowledge of God the Father, through sanctification of the Spirit, unto obedience and sprinkling of the blood of Jesus Christ" (1 Peter 1:2).
...A gathering NOT REVEALED before him...
Some say the coming of Jesus and the gathering of the church taught by Paul in 2 Thessalonians 2:1 and 1 Thessalonians 4:15-17 can't be the same as the coming and gathering already taught by Jesus in John 14:3 and Matthew 24:29-31 because Paul said the rapture and resurrection of the church was a "mystery," that is, that it had not already been taught. But note that all Paul said was a mystery in 1 Corinthians 15:51 was that "we shall not all sleep, but we shall all be changed," that is, that not all Christians would have to die before being changed into their immortal bodies. The rapture and resurrection of all Christians was not a mystery, for Jesus had already clearly taught that: "I will come again, and receive you unto myself" (John 14:3); "And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day" (John 6:40, 44, 54).
...the day of the Lord...
Note that no verse says or requires that the day of the Lord begin before the 2nd coming.
...the time of Jacob's trouble...
What does the Bible itself say "the time of Jacob's trouble" is? (Jeremiah 30:7-8)
...There's NO VERSE that says the body of Christ is on earth during the great tribulation...
Note that we Christians are referred to throughout Revelation (6:11, 7:14, 9:4, 12:17, 13:7-10, 14:12-13, 15:2, 16:15, 18:4, 20:4).