I seem to recall that the Sphinx is defaced because Arab conquerors fired cannons at its head. Is this correct?
No. It was caused from wind erosion.
Two legends about the Sphinx and Napoleon's army in Egypt:
It was French cannon which defaced the face of the Sphinx, almost as a prank.
Napoleon himself said of the Sphinx, "Men, forty centuries are looking down at you!"
Any truth to these?
Yes. As møøslimbs the were offended by the graven image.
Hard to fathom that anything would offend a møøslimb...
The nose appears destroyed on engravings from before Napoleon's time, and given the fact that the French army was accompanied by a group of "savants" (researchers), the target idea is completely ridiculous.
Arab authors mention a fanatic mediaeval “holy man” (Muslim) who set out to destroy pagan monuments. Two traces remain of the lever cavities made to pry off the nose (LEHNER, Compl. Pyr., p. 41 and picture).