Get rid of the V.
Fix a piece of timber to the floor, fix another small piece across the width of the stair/slope, then cut a 5", 6" strip of plasterboard, and screw it to the timber. That'll let the stair slope stop at the new vertical wall/strip you created. Just tape the joint and plaster away. You could even fit a piece of skirting to the new strip to finish it off,, you wont lose much space either. I did a similar job in our house where a shelf met a sloping ceiling, i built out onto the shelf a bit to make it square instead of the awkward V shape,, no probs.
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