Error in both version of the prequel ?

Just wondering....I've only seen Exorcist The Begining version, not Dominion but I heard its exatly the same thing but my question is, is there a mistake in those movies that break the story of Exorsist saga? Why is say that well let me explain
In Exorsict 2, which I wish ill never see again cause it was one of the worst movies ever, we can see the story of father Merin before the events of Exorcist Original film, so is that suposse to be the events of the prequels? But that doesnt work cause in Exorcist 2 many thing are different from the prequels but still are connected and the thing that bugs me the most is that in Exorcist 2, it is the little black boy that is posses by the demon but in the Prequel they make us think its the black boy during all the movie until its actually the other girl that was posses
And Exorcist 2 was made of course before the prequels so are the directors of the prequels the stupidiest persons on earth to change the story like that or its just me that didnt get something and the events of Exorcist prequels and Exorcist 2 arent the same events?