> 2. A way to rotate the before-mentioned sphere around you without forcing the user to rotate it's head. This would avoid much of the neck strain. It could be done by, for example, holding a button on the keyboard while moving the mouse or by a simple keyboard shortcut to rotate the sphere by X degrees in any direction.

one more thing for the author to play around with (I don't have the hardware to try it myself) is experiment with speed/acceleration -- the mouse pointer movement has speed and acceleration parameters that affect how it moves, so that if you want you only have to move your mouse very little in real dimensions to move it thousands of pixels.

It might cause motion sickness, but maybe you can get away with pitching/yawing 5 degrees for every 1 degree of head movement, so to look "straight up" you only have to tilt your head up 18 degrees. Hopefully you'd still have the illusion of being oriented in a space.