Yes, you can use it.
Substituting one for the other would significantly change the taste of the resulting food.
Any white wine would substitute for champagne.
Scuppernong grapes, or muscadine, is correct. Try searching for the correct spelling!
sparkling wine, cava, prosecco
White wine vinegar would be the best substitute, if you have it on hand.
No. Cooking wine does not contain vinegar, and would introduce too much salt.
A muscadine wine is usually sweet because many producers add extra sugar. This is a tradition implemented by the vintners. Dry muscadines can also be found. Muscadines are generally used to make finer sweet wines such as Port, a fortified wine made in Portugal.
around 2.5 lbs to a gallon depending on the grape.
Two different items you will change the outcome of the dish. I would stick to the recipe.
If color is not important, red balsamic vinegar would be the best substitute for white. Otherwise, a sweet white wine would be a possible replacement.
Zinfandel can be substituted for Merlot in either cooking or drinking, although zinfandel has a uniquely discernible taste. There are a number of other red wines that more closely approximate Merlot.
While they both are vinegar, they have very different flavors and it might have a big affect on the overall taste of the finished dish. White vinegar would be the best substitute. Even plain white wine vinegar would be closer.