Hello
I think waiting for The Author to answer and to make rules clearance here seems to be just an unrealistic expectations with low probability to happen 
Well sometimes you roll a 1, and others a nat 20... :-)
But we are All together here, so lets do it on our own.
Below my 3 years old post about that guard rules confusion - as continuation of earlier questions in this topic.Anyone who knows me knows what the past four years of my life has been like, I won't bore you with it here, it's not been good. I do plan to return to Black Ops with a new version in the future.
I guess that some of you play a lot of games of BO, also in gaming clubs with bigger community, so maybe you can give us hints how all that unclear situations were solved in real tabletop battle dust among hundreds of games in a clubs or garages 
Well, let's have a go...
I think that according to the rules you cannot got full control of guard until alarm is raised.Yep.
It is not 100% clear from that, but it rather states that you can only put guard in reserve mode within visual arc.Yep.
Also regarding that guards which survived melee or shooting they can only shout "Intruder alert!Also yep!
"A guard who survives a surprise attack (hit but not wounded) is aware of
his enemy and can shout 'Intruder Alert' when activated, whether he can see the
attacker or not!"
Still not clear, but it is nowhere written that after such an attack you get full control of a guard.You don't.
Read the reserve rules (the Black ops version of 'overwatch'). The figure can react to enemy actions it can see or to a friendly model moving within 6". I wasn't going to have guards just go
"Huh?", forget about it and return to random patrolling.
You can look at it two ways - from a game mechanic or a narrative point of view.
Game Mechanic - Having guards free to activate was too powerful, so the reserve rule was a way to make them very useful but not overpowering (running around after the Black ops team and doing what they liked was too good). So a guard can act on reserve or call out. Normally a good ops team won't leave a guard standing for too long... so when players 'get' the game, it rarely happens.
Narrative - it's cold and dark. Three rounds have zipped past you, one hitting your flack jacket, waking you from aimless thought while on guard duty. You know SOMEONE is out there, and all your concentration is on where the shot came from. You could call out and warn the others or wait to see where the shot came from. What you don't see is the sniper's friend sneaking up behind you.
Hope that helps.
Guy
PS: Thanks to Mark Backhouse for alerting me to this message.