For the first post, you'll need A faction to have at least 5 squads/groups. The invasion commander will at least let one squad/group occupy and defend his initial ground (he will also generate small patrols) then he will pick 2 to 4 groups (one platoon) to move near and approach the B faction OBJ.
At this point, if he lacks troops he will call for more (?) and then launch the "final" assault. Usually i see the AI attacking with one Platoon (4 groups/squads). If B faction commander see it coming and have reserves, he will gather them to come and defend his objective. If not, he will at least defend it with on squad (the squad who initially occupied the objective).
If during the move to the B faction OBJ the attacking troops cross B factions squads, ALIVE will make them virtually "fight". Same story during the approach and the assault itself. Armoured and mechanized have the most value. Motorized is still superior to light infantry. But a weaker troop will always damage a stronger one even if it does loose (to simulate stuff like AT weaponry).
Virtual as real battle damage and casualties are stored and over time units will suffer from this.
Once A faction troops eliminate B faction troops, at least one squad will stay on the objective and will end up capturing/occupying it after some time.
Then of course the front lines will move, you do not really need any scripting to make some basics dynamic mission. It is somewhat the goal of ALIVE. So no worries !
Just remember the AI need 1 squad to defend, 4 to assault, 2 to 4 to approach/go near an objective.
So if you put several objectives on your map, make sure you have assigned enough troops to defend/occupy them all and attack the last of them.
Occupation will defend more, have more reserves but still attack objectives. He will gather troops to defend a menaced obj if possible (not sure how many, but i have seen 5 groups defending a single obj).
Invasion will attack all over the map as long as he have enough troops to do so.
sorry the garbage English ^_^