If I take a reading before I add sugar, the whey is usually around 1.020. I generally get about 1lb 4 oz of cheese from a gallon of milk and about 7 pints of whey. I gotta say that I work hard to get most of the solids out of the milk into the cheese, so that when I make my hard cheese, I cannot get any ricotta from the whey - and if I could that would increase the gravity of the whey. Twenty points of sugar (unfermentable) seems about right.
Regarding the number of lactase tablets I crush and add to the milk before I pitch the yeast, it's 6. I have no good sense of how many I need to break down the lactose in the whey, but I have been going by the assumption that if most people who are lactose intolerant take 9000 units before they eat a dairy meal (3 tabs) then a gallon of why may need 6 tabs... But I could be way off. Rice_Guy may know the answer to that.
I have to say that most wines you make are likely to have a reasonably good mouthfeel , not like eating ice-cream, or drinking a glass of whole fat milk.. but good mouthfeel for a wine. A drink that slides down your throat as it coats you mouth and tongue.