Bind flags can be combined, what you're going to do if your buffer will be both, index and vertex? Not sure why you'd need that but it seems to be possible.
That actually is not possible (that particular combination anyways). But I don't think his design has a problem with this. I personally use a root class of BufferDX11, and then subclass it for each of the types. However, there isn't a whole lot of differentiation between the classes other than the type identification used by the engine. They are there for future additions, which are sure to come.
Keep in mind also that even if you do manage to come up with a buffer that uses multiple flags, you can always just create a new class to represent it that reflects that fact. It will be common to have a shader resource view and an unordered access view together, so you can probably plan on that combination.