While few if any of Melenchon's Hard Left voters will vote for LePen, I don't see many of them backing Macron either, since Melenchon also ran on an anti-globalization platform, albeit for different reasons.
In short, LePen may very well get an upset victory like Trump last November.
I don’t pretend to know how this election will turn out over the next couple of weeks...French politics is, I’m sure, hard enough for those live there to understand.
But I do think one thing of great import has happened. The establishment parties see that they are being tossed aside as failures...so they are repackaging with marketing in order to retain their grasp on power for a little longer. Macron is their new packaging (so like Hillary) along with being a feint (so like Bernie), recognizing that politics there is rapidly changing against the establishment.
LePen will at the worst be the powerful opposition force - always nipping now at the establishment heels. And because the establishment can do nothing but continue on their current path on immigration and EU submission, all the failures, the violence, the economic upheaval, the terrorism and the isolation of the working and middle classes, will be laid firmly at their doorstep with a powerful opposition pointing this out at every turn. Even if LePen doesn’t win, the establishment will need to try to address her issues...or she will annihilate them in succeeding elections.