@ELF Wine, first -- dealing with your current situation. The hydrometer indicates fermentation is done (you already know this). Rack the wine, leaving the sediment behind. When you have less than a gallon, the choices are to top up the container with a compatible wine. Niagara is vitus labrusca, so the result is "grapey". Pretty much any basic dry white wine will probably do.
The other choice is to find a smaller container and move it into that so you have a small air space. From here, let the wine age 2 to 12 months (your own choice on that, personally I'd not bulk age a Niagara for more than 2 to 4 months, but that's my choice). [this is the short answer, as this post is long, I'm intentionally being brief. There is more to bulk aging, but you have time and we need to keep you rolling.]
Next batch? Dump that recipe as it recommends things not normally done with a grape wine.
Start with 9-12 lbs of grapes, crush, press, & ferment. This amount typically gives you ~1 gallon of finished wine. Ferment in a larger, open food grade container. For larger batches a lot of us use Rubbermaid Brutes -- the orange buckets sold at Home Depot are supposed to be food grade and are a size better suited to small batches. Store the excess wine in smaller containers, and use to top up the main container as you lose volume to sediment.
Note: Although it's not typically done with whites, you can crush, ferment, and press (like is done with reds). This produces a much heavier wine -- you may want to try it for fun, as it's only a gallon.
DO NOT add water. Grapes are traditionally the fruit for making wine as grapes contain everything needed to make wine -- fruit juice, sugar, acid, etc. Adding water to a grape wine dilutes it. Other fruits require adding water, most grapes do not.
If the initial SG is below 1.075, add sugar to raise it. SG 1.075 will result in about 10% ABV. SG 1.100 produces over 14% ABV, which may be hotter than you may desire. I typically like my wines to start between 1.080 and 1.090, but this is your decision. You can find online calculators that give you the ABV when entering Brix or SG.
Chances are that if your sugar is low, it is not that low. I recommend adding 1/4 cup sugar at a time, stir very well, then check SG. Repeat until you get the desired SG.
Pectic enzyme and yeast nutrient are good, although you should read the package in case the formulation is different. IMO you don't need to add Campden before fermentation completes, but some like to do it. When using a commercial yeast, it should not impact fermentation. For yeast, the indicated yeast is fine, although any white wine yeast will work just fine.