In don't get why people get hung up about the dating of Merrin's exorcism in Africa. I always assumed that he performed a number of exorcisms down the years.
Why does the exorcism in Dominion/E:TB have to be the one that went on 'for months' and turned his hair white? I assumed that that was a wholly different exorcism after the events of Dominion.
After a number of brutal exorcisms, one assumes that's why Merrin's health diminished so badly in the years between Dominion and the first film!