That’s too bad that you had some stuck fermentations. It’s probable that your yeast died out due to alcohol toxicity resulting from those high brixes. Next time make sure you’re adding enough water and feeding enough nutrients to create as healthy of a fermentation environment as you can. Before next harvest you may want to brush up on stuck fermentation prevention; there have been many articles published in WineMaker magazine on the topic over the years, and some are available by doing a search for the topic on winemakermag.com.
As far as dosing in some of your stuck 2015 wine to future vintages, I have a few things you may want to take into consideration before trying it. Number one is something I’ve been saying for many years and that is, “Never blend a loser.” What I mean by that is: Don’t let a small percentage of a “bad” or troubled wine ruin a future batch. While I understand the temptation to try to salvage something from a winemaking project gone wrong, there are some real reasons why it’s not a good idea. Stuck fermentations can taste and smell bad. You may negatively impact sensory quality of future batches if you mix the two. Stuck fermentations are also notorious environments for spoilage organisms — both yeast and bacteria. It’s possible that you’ll introduce some old “bad actors” like Acetobacter or Pediococci into your pristine brand new wine, thereby potentially spoiling all of 2016 wines.
My advice in this case? Dump the old stuck stuff. You’re only talking about 6 gallons (23 L) of old wine total and in my mind, it’s not worth the risk of spoiling 70 new gallons (265 L) for the sake of re-purposing 6 gallons (23 L).
Really don’t want to dump the old batch? Try adding some brandy and a sweetener to make a fortified wine gift for the holidays. Or infuse in some cinnamon, star anise, citrus peel, and cloves, and then sweeten to taste for a seasonal hot mulled wine.