Sorry, FC, I didn't mean to ignore you! Yes, the aerosurfaces do carry out various functions during the very early part of launch and then position to provide wing load relief. Otherwise all "steering" is effected by thrust vectoring.
Like all space rockets, the shuttle stack climbs initially at a steep angle to get it into the less dense upper atmosphere ASAP, before accelerating rapidly down range. If any abnormal drag had occurred during ascent, the necessary corrections should be evident from the telemetry. I guess by now they have looked through that in great detail.
Any missing tiles or weakened RCC component would probably not be a major factor until the high stresses of re-entry. Whatever first detached over California most likely holds the clue.