The quest info says this is a guaranteed mithril dropper, and amongst other possible drops it lists 5 energy. Not listed on Shibuya's post.

Does the energy drop happen often, or rarely or what? Is it "worth" it?

I accidentally started Elemental Part II after holding off all these years. Now trying to decide if I should abandon other activities to get 180 poisoned maps and 210 poisoned wood. It probably doesn't sound like a lot, but it means a lot of toxic spirit spawns and chopping.

My biggest problem is that the owner of my borrowed ipad is taking it on vacation for 3+ weeks! Using Android cell phone has a 5 second pause every 5 actions, it is just too crushing to bear.

(That's why I haven't played for 2 years. Newer phones and o/s upgrades do not help. My observations indicate the game is primarily aimed at iOS: for example, last android update was in 2019. I don't blame them, designing for both couldn't have been easy. But it's hard for an android user to keep playing.)