1. "variable panel spacing" is simply an US-engineering feature.
2. depress is not during taxi in but starts during descend, valve full open either at preset landing field alt or at touchdown, whatever comes first.
3. no offense, but quite frankly posting about "i heard a noise in a boeing" is a waste of sunday. especially near the wings/main gear bay could be anything, could be nothing. i had a senior stewardess sitting there on her vacation flight. the sound she reported turned out to be the flap drive.... go play.