well dunno bout campaign building (linkin missions)
but thast waypoint thing could be done like this
first there is this thing about AI that keeps firing at the Mgun when the gunner itself is dead.
you can overcome this like so:
make the Mgun an empty unit and name that machinegun...lets say Mgun_1
make a unit, name him, lets say Mgunner_1 and put this into his init field:
Mgunner_1 moveinGunner Mgun_1
then make a trigger, radius not needed, activation none, type switch and put in condition field:
not (alive Mgunner_1)
and activation field: Mgun_1 setcaptive true
this way they stop firing at the gun itself.
now the guys that have to wait:
give that squad a WP where they should wait, and then one where they should go after Mgun is out.
now synchronize that trigger you've made, using F5 mode and drag a line from trigger to the first WP where the men have to wait.
Now they will wait there and if trigger is fired , thus gun is out, they will move to the next.
hope this helps