The paraphrase #5 says "if the specter of choosing a seemingly unelectable nominee does not deter Mr. Trump’s supporters". Does it mean GOP would choose a new candidate to substitute Trump, yet this new candidate still has no hope to become the President?
You meant
paragraph #5. "To paraphrase" means
to restate in other words.
The paragraph does not mean that. It's written at a very advanced level of English, so don't feel bad if you struggle to understand it. I'll paraphrase it.
A little background: the Republican convention will choose the person who will be the Republican candidate in the general election. For the last thirty-six years, the winner has been known beforehand because a candidate has always had a majority of delegates pledged to him before the convention started. This time, there's a good chance that Trump will arrive at the convention without enough delegates pledged to him. If that happens, the convention may become quite chaotic as different factions struggle to get a majority of delegates to vote for their candidate.
Now for the paraphrase:
When the convention starts, Trump's opponents will find it easier to nominate a different candidate if a large number of people who will vote in the general election, and of Republicans, refuse to support him. But the Republican party will suffer even more if Trump wins the nomination after a big fight, despite having little chance of winning the general election.