1. "Activation -> BLUFOR" and "Detected by OPFOR" is a reliable source for detecting the moment when the AI does see
any foe the first time.
ArmA AI is very well aware of it's surroundings, so I'm pretty sure they did see you or someone else because your approach wasn't careful enough.
Using a script with a high
knowsabout value instead of the trigger could delay the alram sound triggering but the ai groups would already be engaging as early like before.
Reducing the enemy skill to 0 should affect their spotting range, though it's influenced by every player's personal difficulty setting and and that why isn't precise.
2.
That's possible by adding a global variable to the last waypoint "On Activation -> wps = true" and creating an end trigger without a radius, activation by nobody and "Condition -> w1 && alive player"
You should know that guiding and meassuring the players progress by waypoints in a mission is inadvisable. Many players do disable waypoint infos in their HUD difficulty so they'll only see them when using tanks or choppers and won't reach them exactly as infantry. When your briefing is imprecise and it does have to rely on waypoints then it's a bad one.
The better way is to guide the player by a good briefing (and maybe some on screen messages) only and meassuring his progress by triggers or scripts with global variables. Then you do combine all these global variables the way you need them in your end trigger(s).
Have a look at the
COMREF for all scripting commands available.