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?

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.