If you think it's a gimmick then don't use it. I personally think it's a good idea. It easily combines the 1D and 1Ds lines and allows users to get that little extra reach if they need it for sports and wildlife photography if they want it. But can switch to FF mode to get wide angle shots or start doing portrait work with the same camera.
The perceived increase in reach is just that, perceived.
And I still think Canon could redesign the mirror motion to allow a EF-S lens to be mounted on a FF camera. But that is beside the point.
Not really possible. The mirror requires X amount of space to flip up. The specifications for EF-S overlap that space by Y.
The only way to avoid that would be to make the top pivot point of the mirror move in addition to the leading edge flipping up. Even then, you're probably only buying yourself a couple of mm. That would result in the mirror being slower, less precise, and induce more vibration; and the more complex the motion of the mirror, the worse each of those attributes gets.