To extend on the question, I had a couple of related ones of my own.
1) Does ambush have to be one of the areas selected for the attack operation (but activating just 1 guerrilla instead of all and treating as if rolled 1)?
Let's see what the rule says:
"4.3.2 Ambush. An Ambush enables 26July to ensure the complete success of an Attack in 1 space. It must take place in a space selected for Attack (3.3.3, not yet resolved) and that has at least 1 Underground 26July Guerrilla (1.4.3)."
2) When attacking, does at least one guerrilla have to be underground in an area prior to the attack?
Not as long as you just Attack, no.
And then the die roll compared to all active guerrillas?
Compared to the Attacking Faction's Guerrillas in the space, all of which will be Active at this point. Active Guerrillas from other Factions don't count.