2) Apple seems like its going to try to sell the main story of the watch as, this will save you time and effort from having to look at your phone all the time. This could very easily backfire unless handled correctly. You don't want to make it sound like your customers are lazy or that they are phone addicts.
3) This is probably the least predictable product Apple has done in a long time. It's not clear that many people are clamoring for this kind of device. If Apple has impressive sales numbers, they will have carved out a market by themselves.
4) What sales numbers will be considered a success? Clearly the upper bound is the number of iPhones out there in the world, which is a ton. No matter how I think about this watch, it feels like an accessory to me and not a stand alone product. You are going to buy this watch to make your iPhone work better for you. If Apple can really sell the message of "If you have an iPhone, you want this watch", they will sell a tremendous amount.