Logical path through Oubliette Sudoku

[CannyK9]

6. Is r1c5 a 2 or a 4? 2.
Here's the grid with the pencil marks from putting 3 and 6 in zone 5:



If I make r1c5 a 4, it forces 239 into the top of zone 3, and now the North 3 must be in either zone 7 or zone 9. Therefore, either zone 6 must be flat or zones 1 and 4 must be flat:



If zone 6 is flat then the North 6s conflict.
If zones 1 and 4 are flat then the grid ends up like this, where there are presumably many problems but at the very least the East 2s and 5s are conflicting:



7. If we fill everything in now, we end up with this:



Because of the North 6s and 3s, both zones 1 and 6 must be raised.

8. Which of the bottom zones are flat? Only zone 8.
Here's what it looks after filling in the cages in zones 1 and 6. The only use of the extra constaint is that the 5 must be away from the edge of zone 6:



That pushes a 3 to the middle column of zone 2, and to the right of zone 8. Now if zone 9 was flat, zone 8 would conflict on East 3s and must also be flat, and the South 5s break:



If zone 7 was flat, the South 7s conflict unless zone 4 is also flat. Now the cage in zone 1 fixes the cage in zone 8, suggesting zone 8 is flat too. This results in a configuration that falls apart pretty quickly:



Therefore only zone 8 is flat on the bottom row. This means that zone 4 must be flat, because neither zones 1 nor 7 are flat.

And this is final configuration of the grid. All of the hard work is done so I'll leave it here~