1. When I was in the cleft and jumped the fence, I did not see anything inside the volcano except for blackness
You can't actually see anything in there (GPNMilano thought about really putting the volcano interior in there, but it turned out to be too hard). So you see nothing but blackness, jump into it, link... and then you reach the inner of the volcano, the same place, just another age (that also solves the problem of the age getting too big).
2. How do you get the MOUL Relto. When I started I went to the TPOTS Relto. I did see in the cleft the 2 relto pages.
As GPNMilano already wrote, a Relto page consists of many parts. Three, actually:
1) The page you click on at the place where you can collect it
2) The page GUI which is shown after clicking on it an which is also in your Relto book (this could also be called two different parts as two different Python files have to be changed)
3) The Relto change itself which gets enabled when the page is enabled
Part 1 and 2 are in the Offline KI for some of the new pages (Cleft, Delin and Tsogal). Part 2 is there for the Er'cana plants. Part 3 is not yet contained for any of the pages, but I plan to add it in a later release (when this one is confirmed to be working and stable), which will make most of the Relto pages completely functional

...I'm impressed. I mean really impressed. Not like a "this is pretty good for someone who never worked on a game before" kind of impressed." It's more of a "I frequently forgot I was technically looking at a hack, and not a beta expansion pack put out by Cyan" kind of impressed.
That's the kind of feedback which makes us going on - thanks a lot

Is it me or is Minkata actually playable? I went as far as the first symbol, and everything seemed to work as it should. So far I haven't found the MOUL K'veer of the Myst V Myst yet, but I was up last night playing it for 4 hours.
Not only Minkata but also Jalak and the Pods should be completely playable and the other MOUL ages completely working

The only exception are the cones in the new Kirel which fall through the ground when you touch them.