Backsweetening should not affect ABV, in that you are doing something to prevent the added sugars from fermenting. This could be achieved by adding a non-fermentable sugar, pasteurizing, adding k-meta/sorbate, or just going over the alcohol tolerance of your yeast strain. But if you aren’t careful, the added sugars will ferment and could carbonate and/or shatter your bottles.
In practice, it’s REALLY HARD to get a mead up to that ABV. Chances are that it will stall around 14% unless you baby the yeast with nutrient and oxygen additions, or step feed it until it slowly makes it up to the maximum alcohol tolerance. I’m too lazy for that, so I figure I’ll just let it get up to 14-15% on its own, and whatever residual sugar is there will be there to stay. Of course, you’ll need to leave it in the fermenter a long time to be confident that it is done fermenting. You really don’t want the yeast to wake back up and decide to finish up whatever is left after you’ve packaged it.
