looking for advice to mitigate inner groove distortion

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.

I hear it mostly in horns and vocals.  It's a fuzzy halo around the sound of the instrument.  Also in loud passages of orchestral works that occur in the inner grooves.  I start to hear it about 3/4 of the way from the edge of the record to the end of the grooves and it gets worse as it gets closer to the center.  It's worse on some records than others and sometimes, I think it's the record that's the culprit but it seems that the turntable and cartridge are playing a role as well.  I checked for anything obstructing the travel of the tonearm but I didn't find anything.  I tried adjusting the anti-skate.  Some fine-tuning helped a little (maybe, not sure I trust my ears sometimes) but it was still there 

What tools are you using to ensure that your cartridge alignment and calibration are correct?

Both channels?  Or one channel more than the other?
cleeds, I think your thought about alignment is reasonable, but as was noted elsewhere, misalignment is not usually that obvious in my own experience and that of others.

Also, where is the motor located on the Rega P3?  Is it under the platter or outboard and perpendicular to a line drawn from the pivot to spindle?