No, it isn't a muddle and, although the position outlined has not been expressly advanced by anyone, I'm saying it's an inescapable logical implication of yours.
Keep in mind that, in the second round, there are only two candidates and two possible outcomes. I can imagine people having cogent reasons for abstaining, but yours seems a little odd. You're basically saying that voting against the FN in this circumstance actually helps them, that defeat is victory, in some unspecified way. I don't see any basis for that, but if I pretend to, then I can't see why it wouldn't be logical to take things a step further and actually vote for them, in order to deny them the second place that would supposedly bolster their position. Unless that's something you would actually recommend, then it seems to me that your position, as stated, is not coherent.