It sounds like the candidate PM would not have the confidence of the Assembly though because the center doesn’t want to play ball with the left and the left doesn’t have a majority.
That’s why I’m suggesting elections. Keep going until either a majority is elected (in which case I assume the president is obligated to appoint its leader) or the parliamentary math changes.
If Macron and the center are serious about keeping the cordon sanitaire against the far right, they should obviously play ball with the left. The fact that they are not tells me that they are not serious. The left should be able to make that argument to the electorate and hope to convince a majority.
Edit: not only is Macron showing lack of seriousness in keeping the far right at bay, he is also undermining the legitimacy of the presidency by playing parliamentary shenanigans and triggering such a constitutional crisis. I never really understood the fundamentals of France’s semi-presidential system, but in a parliamentary republic like Germany, or Ireland, or Greece for example, the president does not get to play politics with the parliament’s confidence like this. I don’t understand why the French think this is a good system.