It has to do with how the sapient beings think of the area, plus influence from how investiture manifests. So kind of both
The Beyond is something else, separate from the Spiritual, Cognitive and Physical Realms, and is an area that the Shards can't touch or see into. Brandon's deliberately left it vague on whether the Beyond actually exists or not.
The inverse landscape seems constant. My theory is that people see land as having a bunch of separate objects on it--dirt, rocks, roads, buildings, etc, so it's a bunch of beads, or mist or whatever objects manifest as. Whereas people see the ocean or a river or a lake as a single object, so it's solid.
I don't think the Shards can affect the landscape very much, other than, you know, being able to rearrange the Physical aspect, thus changing the Cognitive Aspect. The collective thought of masses of populations over long periods of time has more of an effect.
Sel is dangerous because all of the power of 2 somewhat opposing Shards is crammed in there. So they're like a plasma storm--fighting and surging and spinning, random arcs of energy flashing around, etc. I doubt they're affecting the landscape, other than making it perilous.
I agree that Ambition and Odium's fight with Ambition has affected Threnody and its investiture. How, that's still a big question!