Melenchon is definitely not in favour of Russia keeping Ukraine. If I were him I’d take the chance of left unification to silently give up all my previous positions on the Russsia/Ukraine thing. France overall is less hawkish than Macron when it comes to boots on the ground, when it comes to NATO – Remember when Macron called NATO braindead? Melenchon doesn’t like EU austerity politics and such stuff but he’s not an Eurosceptic, he just wants a different Europe. His opposition to a European army was rooted in “an army against what”, again, he should use the chance to make people forget what he said about Russia in the past, if he really wants to get out of NATO strengthening European security integration is the way to go. Though personally I think it’s a good idea to have Europe overall in NATO after all someone has to keep somewhat of a leash on the US.
In any case foreign policy and security is presidential prerogative in France, Macron doesn’t need the assembly to do anything there – and the assembly doesn’t need Macron to do other stuff. If either of the sides is smart they’ll agree to disagree on a couple of things and not oppose each other too heavily, table any remaining issues until 2027 (next presidential elections).