looking for advice to mitigate inner groove distortion

Are you absolutely positive all your adjustments are as good as you can get them?....are you sure, the arm is has free motion....not hitting/rubbing on anything?

I too have an MP200 (along with several other cartridges all premounted on headshells) and experience absolutely no inner groove distortion. Something must be off on your alignment or adjustments.

I would re-align the cartridge, maybe using other methods, and check everything else. This should not happen as you describe it. I would examine the cartridge too.

Audible IGD is not necessarily "part of the deal" with vinyl.  (Tracking angle error is typically greatest at the innermost grooves but this is not necessarily linearly related to audio signal distortion.) Can you describe what you hear and when you hear it?  As in, where is the stylus in relation to the innermost grooves of a typical LP when the distortion sets in?  Is it related to actual tracking or to the distance between the stylus and the spindle?  If the latter, look for something that is obstructing the travel of the tonearm, like a wire that is getting fouled or a problem with anti-skate or with your tonearm bearing.