Spill Bitterness into the Earth

When Volcano Looming High uses Explosive Eruption vs Tsardom of Russia Level 2+, can they Push 1 Explorer from the origin land (when it does damage by destroying Presence) then destroy that Explorer using the thresholds of the innate that deal damage to nearby lands? Does that Push the Explorer again?
No, and no. Actions can't destroy Invaders that were saved from destruction earlier in that same action, so the runaway Explorer is safe. And all the damage is part of a single action, so you only Push them (and generate Fear) once.
The damage from destroying your Presence is part of the same action because the Special Rule says "When you destroy your Presence..." – it modifies how other actions work while they're happening.
Tags: Destroy, Saved From Destruction, Action Modifiers, Damage, 25 more...

Can the Intensify Aspect add multiple token types with Hazards Spread Across the Island?
No. Hazards Spread Across the Island can only add more of the chosen token you are adding, even if other tokens could have been chosen instead.
Similarly, if a Power allows adding multiple token types in combinations you choose (e.g., Utter a Curse of Dread and Bone), Intensify can only add extras of token types that are already being added. And if you could add tokens in multiple places (e.g., Spill Bitterness into the Earth), you can only add extra tokens where you already added tokens of that type.
Tags: Spirit tokens, Action Modifiers, Hazards Spread Across the Island, Utter a Curse of Dread and Bone, 5 more...

When you hit the threshold of a Major Power, is the effect in addition to the base effect?
Yes, unless the card says "instead", threshold effects are in addition to the base effect [p. 17].
(Discussion elsewhere) Tags: Elements, The Jungle Hungers, Tsunami, Vigor of the Breaking Dawn, 27 more...

If you deal damage in multiple lands with a single instruction, does the damage happen simultaneously? Against Russia level 2+, if one of those lands is Isolated, can you attempt to Push an Isolated Explorer and thus destroy it?
Damage instructions are simultaneous in all the lands where they apply. Russia's Sense for Impending Disaster effect requires you to Push an Explorer if you can (and generate 1 Fear); you cannot pick an Isolated Explorer as the one to Push if you have another choice.
This applies to any effect that deals damage in multiple lands at once.
Tags: Damage, Action Modifiers, Bloodwrack Plague, Fire and Flood, 12 more...