The Rod is immovable in relation to what?

[Tanarii]

(3) Because the GM has a consistent worldbuilding including the fact that the D&D universe is literally a post-sci-fy universe (so mankind had an usual sci-fy development, and after breaking the laws of reality in an apocalyptic war you ended up with the D&D universe) and is making sure as many part of the rules are consistent with this worldbuilding (including the fact that the material plane is still made of planets).

There are many "D&D universes", not just one, but I don't think I've ever seen one for which that was stated to be the case. Cite?

Mystara, Forgotten Realms, Darksun, Eberron.

Probably not Dragonlance, the Irda and pre-cataclysm society weren't advanced enough. Not sure about Greyhawk. Definitely not Ravenloft.