How do I solve this joint between interior wall and basement wall

[wallejr]

I have just changed the interior staircase in the 1950s house we bought last fall. The upper floor was added on in 2008, but the construction of the staircase seemed like it was from a funhouse, downright dangerous with small children.

I had a carpenter in who tore down the old staircase, the partition wall against the basement staircase, and the ceiling/floor above the basement entrance to make larger opening dimensions.
Then I got help to open up one side towards the living room of the new partition wall to make it easy to run electrical wiring behind it.

Unfortunately, the whole house is crooked, and right now it looks like this... I wish for tips and advice on how to make this look nice and good.
Should I plaster down to the floor joist or to the edge of the basement wall?
How do I solve the distance between the beginning and the end of the wall, as you can see it differs about 4cm.

I thought about using OSB strips as a spacer but was unsure if there would be too much tension on the plasterboard.
Another idea is if you can hide the seam in some nice way with a seam list or so