Good to hear you are making progress in the right direction.
I misunderstood the way you were moving the spectrum. I too experimented with moving the mirror, though this was 2-3 years ago and I cannot remember all the details. I think I originally saw an improvement in the appearance / reduced height of the spectrum with the mirror at one extreme. I later realised I was getting less flux and so I think I was cutting off some of the light, or missing some of the light in that position.
I will be at the Observers Workshop in September.