(1) Answering Greendragon's question:
I am guessing that to go to the surface we use the Cleft book in Relto and then climb over the fence to where we parked our cars. So I guess my answer is that the instance ends at the Cleft fence, but that fence only went up when Zandi bought the land.IC it's usually vaguely referred to as "going up to the surface," but they never specify how that is achieved. I suppose your relto book will get you to the cavern once you have it, but last time I looked, none of those linking books on my shelf link me back to my house.
(2) We each have a private instance of the Cleft, but such instances are not a natural feature of the surface -- just this one place. Where does the "instancing" end?
(3) If the Cleft is real as is Jeff Zandi, his grill, his airstream -- then over time we have to assume that someone would have taken or broken all that is within the alcoves and rooms. Not the Navajo, of course (their traditional beliefs would label the Cleft as taboo), but certainly the invading Europeans, who in the early days were respecters of nothing that was not of their own making.
I have never found the Atrus time line nailed down to RL specifics. We know that there have been human habitants all over North America for 20,000 years and Spanish conquerors were in the area in the early 16th century. Ana was most likely of European descent. I suppose this is going to be one of those math question (numbers are not my thing ) If Yeesha is alive now (we have seen her) and we know that D'ni live longer than humans, then how long ago was her great-grandmother living in the Cleft. Ana was human and had a human life span, but we do not know if living in a D'ni world changes the life-span for humans (we have seen stranger things).
Here is my proposal for writing about the Cleft in the early 1960s -- and please be thoughtful in your critique:
The Cleft looked very different before the discovery of the Cavern by John Loftin and Elias Zandi. Only then was Yeesha's call heard and we were drawn to the Cleft, which was then instanced and took on the appearance that Yeesha remembered as a child. Artifacts in the rooms, the skeleton and the Riven viewer all appeared. (Bear in mind that if the skeleton and the viewer had been visible for centuries they would have entered written history of the US and since they are not in our books, they must not have been there). So when my archaeologist was studying the area he found the Cleft but devoid of artifacts, the power supply unavailable, no viewer, no skeleton. The wooden bridges would be gone, but he can surmise their existence from remains of rope and the posts.
Viewed in this manner means that any "photos" taken of the site will go through a lot of doctoring to remove items, but I am willing to do it just so you get the best story possible.
Your ideas -- and encouragement -- are always appreciated.