stair tool cutting walls

[Lee66cj5]

I have a stair with open treads on one side for the first 5 1/2 treads then a solid wall hiding the rest of the treads. Under the stairs I have a partial laundry room that has a 2x6 wall instead of the 2x4 wall that encloses the rest of the stair.  I have 2 issues getting the stair to function correctly. the first issue is the 2x6 wall should be cut by the stair and the wall above the stair should become a 2x4 wall (we us a diagonal top plate at the bottom of the stringer height and build 2x4s above). The second issue is if I add a partial height wall below the stair then the side wall cuts below the stringer and makes an open tread stair above which I do not want. Does anyone have any ideas to fix these two issues?

Thanks,

Lee

Marwick.plan

[Lee66cj5]

Thank you! this fixed the problems.