Here are my quick thoughts.
1. Store bought grape juice is made to be enjoyed sweet. When you ferment out all the sugars you lose much of the intended flavor. Of course, you have made wine but the flavor of the alcohol may be overpowering the flavors of those grapes. Try adding some sugar to back sweeten a glass and see if it tastes better. The added sugar SHOULD bring out more of the grape flavor.
You add K-sorbate when you plan on back sweetening (adding sugar) to a wine. So you add the K-sorbate just before you are ready to bottle the wine. You ferment, allow the wine to clear, taste the wine to know how sweet you want it to be, add Potassium meta-bisulfite AND Pottasium Sorbate, then add sugar, and then bottle.
2. I live in upstate New York. I make my wine in my basement and the temperature there is around 60 - 65 F all year round. I don't use a thermometer to check it but if the basement is VERY cold or I want to ferment at a higher temperature I place my fermenting bucket in a large plastic container filled with warm water to which I add an aquarium heater (for tropical fish). If the room feels VERY warm I fill the same container with cold water. But there are wine makers that buy small inexpensive refrigerators and connect them to thermostats that control the temperature of the fridge and so they have complete control over the temperature at which they ferment. I make a gallon or two at a time and the wine I make is for my (and my family's ) pleasure and I prefer to make wine the simplest way I can.
3. Unless you know you have some kind of infection (bacterial or viral) simply sucking on a siphon will not introduce any dangerous or spoilage microbes into your wine. First, the wine is alcohol. Second, you are not touching the inside of the tube. Third there is nothing to prevent you from dipping the tip of the siphon you intend to suck on into say, a glass of alcoholic spirits (vodka?) . Fourth, you can buy self -priming siphons that work without any sucking. And lastly, you can use the siphon you have, fill the tube with water, plunge one end into the wine and allow the water to drain out into another container very close by. This will start the vacuum the siphon needs to work This takes practice.. but it works.
