So about Melenchon's electors going for Le Pen here are the polls in chronological order
Harris : 12 - 13
Ipsos : 9 - 19
Ifop : 19 - 16 - 13
Elabe : 16 - 23
Opinion Way : 15
Odoxa : 19
BVA : 18
Kantar Sofres : 17
The failure to understand that these voters are the ones who would've voted for FN in the first place is staggering to say the least.
Melenchon didn't go from 11 to 19% in 5 years with pure magic, the additional voters he convinced were mostly coming from both the PS and FN, these 15-20% is 3-4% of total electors, this is basically the amount of voters that intended to vote Le Pen originally (remember these polls with Le Pen at 25-30%, who do you think they voted for if not Le Pen in the end?) and if anything his stance maye have convinced a good part of them to not vote Le Pen at the 2nd turn.
He's also clearly trying to keep these people on board with the Front de Gauche for the incoming legislatives, while it may hurt Macron in the short term, in mid/long term it increases its potential electors and also put these people away from the FN which might also be a top priority for him.
But yeah he could have joined the Front Republicain, bringing Macron from 60 to a whooping 63-64%, lose a few deputees in the following election, lose the FN electors he just won after the presidential campain and let them go back to Le Pen once and for all.
The supposed "short term gains for democracy" just aren't worth it.
I'm only speaking from a political point of view there though and trying to guess what is Melenchon's and the FI thought process, I don't judge it nor do I necessary agree with it.