Let's say 3 levy infantries swarm one enemy unit. And the opponent plays First Strike/Ambush. Does he get to choose which of the 3 units he strikes first?
(RB) Yes

If one of the 3 levies is eliminated by the first strike, is the entire swarm attack called off, or do the remaining 2 units still combine their attack dice and attack without the unit that just died?
(RB) Yes - the remaining 2 units still combine their attack dice and attack without the unit that just died

Can the defender choose the target of the battle back?
(RB) Yes