signal clipped despite q point is middle
You've just discovered that the AC load line is different from the DC load line.

Although the amplifier swings about your DC load point, the disturbance does not follow the DC load line.

Generally speaking, the AC load line crosses the DC load line at the Q point, but is more steep, so it intercepts the horizontal axis at a smaller voltage than the DC load line.

To maximize the AC swing, you have to relocate the Q point.

The different slope is because AC sees different impedances. The impedance of the base-emitter junction of the transistor is different for AC. The impedance of the emitter branch is different (because of the bypass resistor).   To AC, both the positive rail and ground look like a ground.

AC and DC load line video: http://www.youtube.com/watch?v=lMPPDpia2lc