I totally appreciate your sentiment and generally agree, but with the caveat that the problem you’re facing is not just a problem with this current election, but an inherent issue with your electoral system.

This is a bit of a misleading summary.

Melenchon speaks for his own party, France Unbowed (LFI), not the entire NFP alliance.

The NFP as a whole has not declared support for Melenchon’s position, although his party controls 71 (~41%) of NFP’s 180 seats in the National Assembly.

Macron has already indicated that he will not allow Melenchon to become Prime Minister, and the entire NFP will be aware that they must select a more moderate leader to represent them if they expect to gain enough support from the centre to operate as a minority government.