If the RN wins an absolute parliamentary majority in the second round, it will be entitled to govern, but only within the limits created by Macron. There are two possible results to this unsavory cohabitation: either the RN will make a mess of it, in which case they will be punished during the next presidential election; or they will find success in enforced moderation and have an incentive to continue with it. Either way, the Macron training wheels will keep them under control.
I suspect that was Macron’s thinking when he opted for the snap election. Let’s hope he was right.