Regarding rarity and power level, I don't think the lantern really qualifies as an artifact by normal D&D/Pathfinder conception. It was one of a kind originally, but really it's not that mind blowing. What ends up being powerful is the continental network of lighthouse sized lanterns that help stabilize the transition to the new planar order and subdue the populace in act three.
I think your design (and the attunement effects in your GM's guide) work well. I think you could concoct a reason for the minor lanterns not being so strong. Something like:
"These lanterns aren't quite as powerful as a normal wayfarer's lantern, and cannot overcome natural local energies without a lot of preparation. A character can calibrate the lantern to a specific location by spending 8 hours and making a {insert appropriate check for PF2 here}. Until that time, the lantern can produce no magical effects, and if it is moved more than 30 feet, the calibration is disrupted."
I believe my original idea for the lanterns in the glacial rift was that they were using oil derived from meteorites that landed from worlds other than the ones that orbit in the star system. The researchers tried a whole slew of rocks they found out on the glaciers, and when they discovered the plane that does the pacification, they were able to locate it with divination and point a planar telescope at it to infuse more oil, mass producing the stuff for the Ob conclave and later the big lighthouses.