Exaltation of the Incandescent Sky

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...

For Powers that let you skip some # of Invader Actions, how/when do you decide which Invader Actions to skip?
You decide when they come up.
You can skip as many as you want, up to the limit given. You are not obliged to use all skips to which you are entitled (you could always be saving them for a future Invader Action caused by something unusual). Unused skips do not carry over to the next turn (as normal for effects that don't change pieces).
(Discussion elsewhere) Tags: Skipping Actions, A Dreadful Tide of Scurrying Flesh (Many Minds Move as One), Infestation of Venomous Spiders, Exaltation of the Incandescent Sky, 5 more...
If a Spirit-targeting Power adds Fear, which land does it add Fear in (e.g., for the purpose of Dread Apparitions)?
No specific land, unless the Power explicitly says which land to affect.
Mantle of Dread and Exaltation of the Incandescent Sky notably don't specify where the Fear is earned, and thus can't add Defend with Dread Apparitions or trigger a Push from Stretch Out Coils of Foreboding Dread.
Tags: Fear, Dread Apparitions (Bringer of Dreams and Nightmares), Foreboding Aspect (Shadows Flicker Like Flame), Mantle of Dread (Shadows Flicker Like Flame), 1 more...
Can the threshold of "Exaltation of the Incandescent Sky" skip multiple Invader actions in one land?
No. You choose four total lands at once, rather than choosing a land four times.
Tags: Skipping Actions, Exaltation of the Incandescent Sky

When playing Ward the Shores, if you play a Power Card not during the Spirit Phase, can you play it face-down to add a warding pattern?
Yes, subject to the "once per turn" limit.
(Discussion elsewhere) Tags: Playing Power Cards, Ward the Shores, Spur on with Words of Fire, Gift of the Primordial Deeps (Serpent Slumbering Beneath the Island), 4 more...