You're conflating the two scenarios with two rounds of ranked voting allocation.
The scenarios aren't the same instance of voting, they are separate instances of near-identical voting (think of the first scenario as polling and the second as the actual vote, if it helps), intended to how how both IRV and STV in single-winner elections a voter's bottom preference can win even because they, and only they, switched the rankings of their first two preferred candidates.
You're absolutely right that candidates need to make their case, and that IRV opens the door to 3rd-parties more. But it comes with an inescapable mathematical fact that you can vote for your preferred candidate, and cause that candidate to lose. Adopting IRV or STV for single-winner races means having to live with that.
I'm still not understanding what is meant by "voting for your preferred candidate and causing them to lose". How is that even possible? If your first choice passes a round, then your vote helped them move past that round and remains with them in the next rounds until they are eliminated or win. If your first choice gets eliminated, it's not your single vote that made them lose the round (you tried to help them win the round!) but the fact that they didn't have enough overall supporters and came in last place in the round and were eliminated, so your ballot then goes to your next choice.
There's really no way to reasonably convince people to vote for your side second as a specific outcome unless you only intend to come in second. C would have been much better off trying to convince the B-voters who put C second to put C first than the A-voters who put them last.[DOUBLEPOST=1476401130,1476400973][/DOUBLEPOST]This is why Ash is saying he likes the Borda Count ranking method. This particular scenario we're talking about cannot happen with that method.
Okay, let me work through this statement: "C would have been much better off trying to convince the B-voters who put C second to put C first than the A-voters who put them last."
If we assume B and C are major parties (because they made it to the final round) and A is a third party...
The statement is saying Republicans are better off trying to convince Democrats who put Republicans second to put Republicans first, rather than the Libertarians who put Republicans third in their list.
But Republicans and Democrats are most likely not going to be eliminated from an IRV round (except the final one to select the winner) at this point in time. So it doesn't matter what a Republican or Democrat supporter puts as their second, third, etc. choice, because until third parties become stronger, Democrat and Republican candidates are not going to be eliminated in IRV rounds and voters choosing them as their first choice will always make it to the final round with their first choice. Until things change with our political parties, Libertarians, Greens, and other third parties would be the only candidates eliminated in each of the rounds and have to have their ballots go to second, third, etc. choices.
So it makes sense for Republicans and Democrats to listen to and court third parties for their second choice votes in IRV, as those are the candidates most likely to be eliminated and have to go to the other choices on the ballots. Which is going to increase the say third parties have in the political process and allow them to (hopefully) grow to be real competition for the major parties.
Also, Republicans courting Democrats (and vice versa) doesn't work so well under FPTP, so I don't see it working so well under IRV either.
To use a saner election as an example, a Bill Clinton supporter may have been okay with Bob Dole winning, but I doubt you could have convinced them to put Dole first on an IRV ballot.