Under the stairs - 3 problems

[rsodar]

1. Little problem with my stairs, actually beneath them. As you see on the picture atached this are U-turn stairs. I need to close hole by extending wall beneath but when I do that winders for stairs don't seems to work.
2. I have this hole on stairs (from this side I have put invisible wall for winders to work correctly) and it seems this anomaly starts where wall underneath stairs stops - or it is by chance.
3. How to hide stairs but to have room beneath to the wall under stairs?

Thanks in advance