For Monaco the real test will be the Europa League because I highly doubt Nice would cause them a big trouble seeing their super recent form going from win to win.
Marseille is another team who are keeping up very well for the title fight against Paris and they are just 5 points ahead.Paris has a really strong test if not the most difficult of all season when they will be going next week to play in Munchen against Bayern,this trip most likely will end up badly for them but since they have super star players I say hope dies last here.The title of the Ligue 1 though will be disputed between Paris,Marseille and Monaco as I don't see any other team at the moment to be able to fight for the title.
I'm not sure, because Nice is currently in good form, they are unbeaten in their last seven matches in Ligue 1. Unlike Monaco, before the match against Nice. Monaco will play against Leverkusen at Stade Luis II, their own headquarters.This match will be quite tough, because Leverkusen will play all out to win the match over Monaco. which in the end, the Monaco players could have experienced fatigue when they played against Nice.
On the other hand, Marseille has a great chance to overthrow PSG at their own turf. especially if PSG are not strengthened by one of their star players, Neymar who has been hit by an injury. Marseille must take advantage of this momentum, to reduce the difference in points from PSG.
As for PSG, as you said. currently the Galtier troop squad is in an unpleasant situation. they have a tough match, apart from against Marseille, Galtier must prepare his team for the match against Baryern Munich in the second leg later. even so, if only for the Ligue 1 trophy, I still believe PSG's dominance will be difficult to break.