I'm going to have to do a lot more reading on this again but from what I remember reading before (can't remember where, which is why I'm going to have to go back and study some more), the first part of all these plagues (which might be what we call the Tribulation) is going to be God trying to get people's attention and get them to turn to Him. Remember, that's what He did with Israel; when they grew lazy and wicked because of all their blessings, God allowed them to be conquered and captured by their enemies and turned into slaves. When they couldn't bear the oppression anymore, they turned to God and He rescued them. This happened over and over until God finally had enough and scattered the northern kingdom of Israel altogether when they were conquered by the Assyrians. Judah lasted a while longer until they were captured by Babylon.
The two witnesses in chapter 11 will be preaching for 42 months (3 1/2 years) and then the Beast which appears in chapter 13 will be given power for another 42 months. (Maybe this is where the idea of the tribulation lasting 7 years comes from.)
The part about the first reaping reminds me of these Scriptures:
Matthew 9:35 And Jesus went about all the cities and villages, teaching in their synagogues, and preaching the gospel of the kingdom, and healing every sickness and every disease among the people. 36 But when he saw the multitudes, he was moved with compassion on them, because they fainted, and were scattered abroad, as sheep having no shepherd. 37 Then saith he unto his disciples, The harvest truly is plenteous, but the labourers are few; 38 pray ye therefore the Lord of the harvest, that he will send forth labourers into his harvest.
Luke 10 After these things the Lord appointed other seventy also, and sent them two and two before his face into every city and place, whither he himself would come. 2 Therefore said he unto them, The harvest truly is great, but the labourers are few: pray ye therefore the Lord of the harvest, that he would send forth labourers into his harvest.
I don't know that Revelation is talking about the same type of thing, though.
I do believe you were correct in your first assumption, though, that eschatological beliefs really have nothing to do with salvation. It's just that the same people who have false assumptions about end times also have false doctrine about nearly everything else, including salvation. If someone has the correct idea about repentance and conversion and is genuinely saved, they're not going to go to hell because they believe in a pre-trib rapture.