Spells and Artifacts can not be used in defense. Overall, this works just the same as a Summoning Spell. There is no Cosmic Battle in this case, so special effects with that specific wording have no effect. To all these questions, the answer is yes. If a Location is revealed, treat it exactly as if it had just been played as a Current Location.
Sanity is gained or lost.
The gate is usable. The Location still becomes your opponent's Current Location. Other restriction are irrelevent. The Vortex special effect overrides them. The Monster never properly entered play.