Ubiq: If you write in a candidate, you’re voting for him to be an ELECTOR; you’re not voting for him to be President. Furthermore, most states require the electors from that state to be a resident of that state.
Mike Pence isn’t a resident in most states.
“why not nominate a republican who will win a general election (and lacks Trump’s “despicable mannerisms.”)”
Because that candidate you’re describing isn’t on the general election ballot for President, as a viable major party candidate. And either wasn’t a Republican party presidential primary candidate or lost the Republican primary election.