I don't believe it is fair to rate "Grow Up, David Addison!" in its current, incomplete state, and I have inquired as to whether or not the author has any interest in finishing the story. The concept is unique in the Moonlighting EU, and it has the interesting potential of playing out like a backwards Big, with incredible circumstances making it easier for both David and Maddie to reflect on and open up to each other than it ever would be otherwise, especially during its season 2 timeframe.
Has the author gotten back to you yet about finishing this? I always loved the movie "Big", so this could be interesting. I would really like to know how this situation might make Maddie see David in a different light.
Unfortunately, I have not yet heard back from the author, and I suspect the odds of that changing are slim. "He" (I am assuming from the profile photos) has not touched this particular story since 2013, nor posted anything else at FanFiction.net since 2018. Never say never, though.