Yes, if she didn't add anything but sugar. However, the cooking will have seriously affected the quality of the wine which you can make from the juice. [It won't be as good as uncooked.] Also, if the added too much sugar, you won't be able to ferment it either.
That would work, but would if the sugar content were low, the wine would be low in alcohol, and wouldn't keep very well. Conversely, if the sugar content were high, you might not be able to start the ferment, or the wine might end up to alcoholic.
Bread yeast will not ferment to more than about 9%, and it won't eve start in a high-sugar juice. You need a good wine yeast for wine.
Yes. The more sugar, the more alcohol --- theoretically. However, if there is too much sugar initially, the yeast may die immediately, or they may convert some of the sugar to alcohol, and then die because of the high alcohol environment. Generally, one wants 20-24% sugar initially.