What are the principal advantages of using a single building information model of the existing conditions as the foundation for modeling proposed additions or renovations?

It's a good way to make sure you understand all existing conditions (because you're forced to use them as you update the model), and it's also a great way to keep track of how the building is changing with time; especially because it's easy to generate demolition models of the demolition and new construction.

In your project, which features of the proposed design did you choose to model as design alternatives?

I decided to model different facade finishes as the design alternatives, because I was actually between two ideas when I started modeling, and I personally wanted to see how they would both look. As I created the design alternatives, I kept getting stuck on how to edit certain features; Revit didn't let me enter walls with arrayed elements into sets, so I eventually gave up on that and varied the facade on the first floor only.