yes for your ABV calcs you need to add the maple syrup to the original gravity. I call it Calculated original gravity.
Adding the maple syrup later as opposed to right when you pitch probably will help you attenuate better, that's kind of the point. By adding the maple syrup later you are #1 giving the yeast a better (lower gravity) environment to ferment in and they'll stay healthier, #2 (as you mentioned) it makes them eat the tougher sugars first, then they get the easy maple syrup sugars.
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