Pyroclastic Flow

What does it mean when a Power's effects or Target Land shows (Terrain A)(Terrain B)? A or B? A next to B?
It means "Terrain A OR Terrain B." (Back page, Targeting section.)
The division between them is supposed to be a sort of slash, but that's not especially clear.
(Discussion elsewhere) Tags: Targeting, Terrain, Iconography, Uncanny Melting, 35 more...

Does a Power stop resolving if you can no longer meet the Range/Target requirements in the middle of using it? (e.g., because it moves Presence out of the origin land)
No. Range and target requirements are only checked as part of targeting the Power, before resolving any of its text effects.
Tags: Range, Targeting, Timing, Ordering, 24 more...

If the Incarna of Towering Roots of the Jungle is destroyed in the middle of an action, do Invaders/Dahan/Beasts become vulnerable immediately?
Yes, those pieces are vulnerable to any Damage or Destroy effects after that point. Usually the Incarna is destroyed by choosing to count it as a Presence when Blight is added.
To cover some of the common situations:
- Ravage (without other effects): The Dahan are protected by the Incarna, since it was present when they were dealt Damage. But the Incarna is gone during the Dahan counterattack, so the Invaders take Damage normally.
- "Draw Towards a Consuming Void," "Drought," "Poisoned Land," "Pyroclastic Flow," "Pillar of Living Flame," "Volcanic Eruption": The Incarna is destroyed at the end of the base effect, so only the threshold can successfully deal Damage or destroy anything.
- "Insatiable Hunger of the Swarm": The Incarna is destroyed at the beginning of the power, so doesn't protect anything.
- "Cast Down Into the Briny Deep" threshold: The Incarna protects the Invaders/Dahan/Beasts in its land before being destroyed. Those pieces are then cleaned up for being in an illegal location.
  Tags: Ravage, Incarna, Draw Towards a Consuming Void, Drought, 8 more...