NFP has said that he won't be their PM candidate. My suspicion is that it will have be someone from either PS or EELG in order to bring the Macronists along.
There will be a lot of horse trading. The NFP will need to form a parliamentary block large enough to reach a majority. In doing so, Melenchon will be a hurdle, imo, to reach that majority if he puts his name out there for PM.
So the PM will have to be a consensus choice. Yes, a PS is very likely.
I do wonder if Melenchon winds up putting his name out there after all just to make taking it back off the table something to "give" to the Macronists in the negotiations in exchange for policy concessions.
One more thing, Melenchon might not want to be a PM bc that’s a poisoned gift.
He will for sure run for president, & he doesn’t want his popularity to be tarnished by being a PM