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I think you want to negate the axis. Finding the inverse of the rotation matrix is the matrix that when multiplying the original matrix returns a 1.

Why are you negating the (x,y,z) components of the quaternion?

scanbooster is not only negating the axis, he/she is also permutating y and z

If I remember correctly, the inverse of (x, y, z, w) quaternion is (with S being square of norm, S = x2 + y2 + z2 + w2):

(-x/S, -y/S, -z/S, w/S)

So maybe there is an error here:

self.init(ix: -Float(cmqUpright.x), iy: -Float(cmqUpright.z), iz: -Float(cmqUpright.y), r: Float(cmqUpright.w))

Shouldn't it be

self.init(ix: -Float(cmqUpright.x), iy: -Float(cmqUpright.y), iz: -Float(cmqUpright.z), r: Float(cmqUpright.w))

of course assuming cmqUpright as a norm value of 1.

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BTW, was the answer OK ?