Hmmm, If I understand correctly, your first question was already answered?
1)If not, just place a big enough trigger to cover the area that you would like to have effected by the alarm and set it side detected by side depending on which side you want to activate the trigger. And set it to repeatedly. In the effects tab of the trigger you will have to find the sound that plays the alarm
2)I think you will have to add an objective which orders the player to set mines out on the road and use a marker to identify where(if you choose).
Or you could add a custom sound file where the unit speaks to the player directly and orders him to perform the task and add text on screen with the Hint command i.e Hint:Place some mines on the road near that intersection OR Titletext ["Place some mines near that bridge head", "Plain"] and make sure the player has access to mines.
3)One way is to use ...unitname addrating -10000 (thats negative 10000)...but this will make him enemy to all. The better way is to group your civilian prisoner to a west leader(or east if the enemy is west) and in the init field of the leader put this...deletevehicle this...This will remove the leader and leave the civilain as a west unit(or east if you used an east leader). You then have to put this in the units init field...this setcaptive true...The last thing you need to do is place a trigger which covers the area that you require the unit to stay inside of and then group thr civilian unit to that trigger by dragging a line from the unit to the trigger in group mode and set the trigger to...not present... In the Onactivation field of the trigger put this code...civyunitname setcaptive false...civyunitname is the name that you chose to call the civilian unit. There is a way to do a group of civilains like that all at once but I do not know the code to isolate and setcaptive false only the unit that flees the trigger. Perhaps another can chime in here and provide that code on this post as it would benefit future readers.
4)If you are wanting a chopper to hover at ground level to allow units to get out? Then in the waypoint of the chopper in the Onactivation field insert this...choppername flyinheight 0...If you need the chopper to hover at building top level it will require more work such as...placing a gamelogic to identify a specific position, testing to figure out the exact height of the building top so that the chopper dopes not collide with it, and a small script which loops and checks for the choppers position, speed, or height while nudging it toward where it needs to be. To have the AI units getout when the chopper is in position you will have to create a trigger which checks the choppers speed height and/or position before ordering them out with the action command soldier1 action ["eject",choppername] as for the code that checks the speed, height or position? someone else will have to help you here or you can break open other members missions and see how they did it.
Hope this at the least sets you in the right direction?