I think I still have no idea what the hell a caucus is.
If Trump doesn’t win Iowa, Iowa isn’t relevant.
It's like election day, except the polls are only open for the people who attend a meeting. Everybody who cares to shows up for the meeting, some speeches are given, then they all vote (pretty much "at the same time" in any given meeting) and go home.
With an election, each person shows up at the polling place sometime/anytime in the day, at the polling place, goes through the gauntlet at the door, votes, and then goes about their business.