The Rod is immovable in relation to what?

[Greywander]

As an aside - sea level is a really crappy reference point IRL as it is nearly undefinable / not fixed.

Unless it is. See my previous post about how appeals to realism generally fall into either "like real life" or "internally consistent". No reason to think sea level would need to change in a fantasy world. Maybe it does, maybe it doesn't. It all depends on the world building.

Sure it is. The elevation where pressure is 1 atm. Easy peasy, lemme know what else you need defined.

The same applies here, but in the other way. This is fantasy, not science fiction. You need to define things in mythological terms, not scientific ones. Something like "pressure" might not even exist in the same sense. After all, imagine how much you need to bend the rules of science to break the square-cube law and get giant creatures that don't collapse under their own weight, to name but one aspect of a fantasy world that breaks the laws of physics.

The idea mentioned early about using the highest stone from a mountain attuning the rod to that mountain is more in line with how fantasy typically works. It does still beg questions like what happens if the mountain gets split in two, or gets leveled, or whatever, but you can probably work those out. It is at least approaching the concept from a mythological perspective, rather than a scientific one.