While OED does not specify which “a-” this is, it does tell us it’s from “on middan”, so my money is on preposition 1 (in the situation).

• 03.050 Now they rode away amid songs of farewell
• 10.037 amid scenes of astonishing enthusiasm.
• 14.012 Amid shrieks
• 14.043 amid the ruined piles of the old town.

“amid, adv. and prep.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.