Excellent thoughts. I did consider that with Satellite technology, we've essentially taken the mystery out of pretty much everything in our world today.... but there's a great deal of latitude between a culture that may or may not be on the cusp of developing gunpowder and one that has satellites in the sky.

One of the things I've thought a bit about is how the ebb and flow of disasters and cataclysms, plagues and barbarians, etc. effect the development and loss of technology. I think I also need to think about how that technology flows across cultural boundaries, and how difficult or easy it is for something know in one place to reach another place.

The same is true of cultural developments and changes.

With regards to the mysteries of past mythologies across multiple cultures, the answer to that is easier in a fantasy context: it's because the myth is based on something that's real! (Sometimes that's true in the real world, too, of course).

Sure... we can't necessarily experiment with these things scientifically... but we can at least run thought experiments in our heads to reason out how, logically, these things may have come to be. That's the kind of thing I'm thinking about with regards to how to approach world-building.