Why would we go somewhere else though? Because right after the Lord returns in a Post-Trib Rapture, He conquers the AC and Satan and the opposing world to establish the Millennial Age. I can understand if Jesus takes us up if the rapture was before the Great Tribulation to wait things out, but in a Post-Trib Rapture, the Great Tribulation had already passed and therefore no waiting around require.
Where do you get idea that the 144,000 are going to spread the Gospel message? Because if you read Revelation 14, it is an angel who spreads the gospel:
Revelation 14:6 Then I saw another angel flying in midair, and he had the eternal gospel to proclaim to those who live on the earth—to every nation, tribe, language and people. 7 He said in a loud voice, “Fear God and give him glory, because the hour of his judgment has come. Worship him who made the heavens, the earth, the sea and the springs of water.”
The 144,000 are actually the firstfruits, but no mention of them spreading the gospel.
Revelation 14:4 These are those who did not defile themselves with women, for they remained virgins. They follow the Lamb wherever he goes. They were purchased from among mankind and offered as firstfruits to God and the Lamb.
I understand "keep thee from the hour of trial" to simply mean that God will keep our salvation in Him safely, not so much as take us out of the situation. Just like with Noah who God didn't take out of the Flood, but got Noah through it. It can also be said that even if a faithful, enduring Christian dies in Christ, their salvation remains intact as opposed to the unfaithful Christian that apostatizes (falls away) from the Lord.
After reading your post, I don't really know how to reply to it. You have so many things going on in it. Some I agree with and others, not so much. So I will attempt to slow it down a bit.
You are correct in saying that the Pre-Trib. Rapture is the one that carries some sense of purpose. You said:
"..I can understand if Jesus takes us up if the rapture was before the Great Tribulation to wait things out..." It is not so much that we are being taken out so we can wait things out but to be kept safe from the judgements of God being poured out upon the world and the hardships these create. In this same way, Noah and the seven with him, rode safely above the wrath of the flood. God placed them in the Ark of safety. God closed the door behind them. They looked not upon the judgement of God which befell the whole world.
The 144,000, who are sealed Jews, for God's purpose during the Great Tribulation, are indeed witnesses. For who has ever heard of someone being "sealed" and not witnessing. Christians are sealed by the Holy Spirit, (2 Cor. 1:22; Eph. 1:13), and one of our principle works here on earth is to witness. Why, would you think these 144,000 would not witness for Christ?
As to Revelation 3:10, and it's promise - we need to take note of two things and this will require getting into the original Koine Greek to see the full understanding of this verse. You posted this:
"I understand "keep thee from the hour of trial" to simply mean that God will keep our salvation in Him safely, not so much as take us out of the situation."
"..keep thee from the hour of trial...", is
"τηρήσω ἐκ τῆς ὥρας τοῦ πειρασμοῦ". The first thing I wish to point out, is the Greek word
ἐκ, which in most Bible translations is translated with the English word: "from", and this is OK; however, we need to understand the full implications of this Greek preposition. It carries the meaning of: "out of", as in taken out of or in our case "keep thee out of".
The second thing is the Greek word translated "hour" -
ὥρας. Which carries the meaning of "a determined period of time". It has been translated with the English words: "hour", "day", "season". Here are a few examples, of it's use in Scripture, where it does not mean "hour":
Mat 14:15 And when even was come, the disciples came to him, saying, The place is desert, and the
time is already past; send the multitudes away, that they may go into the villages, and buy themselves food.
1Th 2:17 But we, brethren, being bereaved of you for a
short season, in presence not in heart, endeavored the more exceedingly to see your face with great desire:
John 5:35 He was the lamp that burneth and shineth; and ye were willing to rejoice for
a season in his light.
If we put this all together... then we see that the promise is to keep believers "out of" this coming period of trial. Remember also, this is not just any "trial", this is the one that ".. shall come upon
all the world, to try them that dwell upon the earth."
Only a Pre-Trib. Rapture can accomplish this purpose and mirror the OT safety of the Ark. The "Universal" church of believers shall be riding high above this which is to come upon the whole world.