1. Yes, going from the beta to the full release will simply be an update.
2. Laminar Research doesn't have a timeline yet for how long we will be in beta. We've been in beta 1 for almost 2 weeks, so I believe it will be a while still (again, this is just an unofficial guess, it really depends on many factors) before X-Plane 11 is final.
3. This is very subjective. :) X-Plane 10 will continue to work, even after X-Plane 11 is final. You can use it for many more years if you wish. It depends on how badly you want to use the new features, such as the tutorials, new UI, FMOD sound, etc, and how much tolerance you have for bugs and problems.