The most hilarious potential run-off would be Melenchon and Fillon, and I'm beginning to suspect that is the outcome. I really can't predict - but Macron's numbers have been slightly more steady than Le Pen, who seems to be draining support to Melenchon and to Fillon.
Le Pen's support hasn't changed since the start. She had 89% of her voters who said it was certainty they were going to vote for her. Her share going down is because of a sudden undecided swing towards alternative candidates.
Hence why the media are rightly point towards Macron losing voters. He still has a lot of undecided.
Would Le Pen voters even turn out in the runoff if it comes down to Melanchon and Macron?
A significant number would vote for Mélenchon (he is anti-establishment as Le Pen).
I'm not sure its a significant number and I doubt it would be enough. Remember that FN fought years against the PCF. Her core 15-18% will not touch someone like Mélenchon - the rest would more than likely stay home. Overall in the transfers only about 20% of her entire voter base would transfer to Mélenchon IIRC (the other thread with the figures got deleted). Macron actually does better as a second choice for FN voters than the others.
It would certainly make for an interesting FN internal fight as to what to do but if Marine doesn't make the second round she resigns immediately.