Back to work…
1) The consensus here seems to be that merely defeating Malfoy is enough to gain the allegiance of the elder wand. This does not resonate with me for two reasons: Disarming him of a different wand is hardly “defeating” him, and this also must assume that no one had disarmed Malfoy in the many-months-long interim.
It’s possible that it’s not enough to ‘defeat’ Malfoy. What was suggested above was that by conquering his wand, and changing its allegiance, he takes possession over any wand under Malfoy’s ????. Remember, she makes up the rules.