1) To count the number of alive units in an array do:
nrAlive = "alive _x" count myUnitArray
where myUnitArray is something like:
'[unit0,unit1,unit2]', or 'units group player', 'thislist', 'list myTrigger' or whatever.
2) Check the 'velocity' command, it returns the speed vector of an object/vehicle.
Depending on how the missile is made, it might not work, but thats where you wanna start digging anyway.
Syntax not guaranteed.