Why is that contrived? That's not really far-fetched that having a Daedric Lord's blessing will help you. Because as the story goes, there is no priests or worship of Nocturnal, she basically gives you luck and an edge.
Better than Obsidian going into hyber-bole science trying to explain how a GOD-like being is able to give you some luck. That is sooooo far fetched, sarcasm. She's a Daedra for Devil's sake.
Because, Mercer really isn't all that extraordinary? I mean, even if you fight him at level 50+ he's still using a dwarven sword of all things. And despite being able to use all three Nightingale abilities on a much shorter recast than is possible, he still comes off as a weakling.
That's not even taking into account that this plot point could be forced onto us while our character has already possibly defeated Alduin, an actual near physical deity. Or are we to assume Mercer is somehow more difficult for our character to put down than a demi-god in the shape of a dragon?