looking for advice to mitigate inner groove distortion

Apparently the OP is on vacation.
tz, You have successfully come up with an explanation for the OP’s problem that is even more unlikely or far-fetched than my own or any of the others. Are there any data suggesting that stylus shape and alignment algorithm are linked in any way? If so, I am willing and anxious to learn about it.

Cleeds, I don’t think we are listening to each other. I am saying that I do not think that the particular symptom reported by the OP is due to misalignment. I did not say that misalignment could not cause distortion, although I have my private doubts about the audibility of such distortion. You say that misalignment can be detected on a distortion analyzer. Do you know of any published data to that effect? I would be interested to read any article about that, but I have been unable to find such an article after doing a computer search. By the way, all three of the popular alignment algorithms result in some degree of tracking angle error at the inner grooves, as you probably know. Of the three, the innermost null point for the Stevenson alignment is closest to the spindle. For that reason inner groove distortion with the Stevenson alignment is probably not worse or significantly worse than that of any of the others.But this is a minor point. I agree with you that most of us have made a tiny error here and there in aligning our cartridges. That means nearly none of us is listening to a perfectly aligned cartridge.

I think that the OP might be experiencing an effect of the Rega motor on his cartridge due to EMI. The fact that it gets worse as the cartridge approaches the spindle is consistent with that, just as it is consistent with inner groove distortion. Furthermore, it seems to me that I recall that Rega motors are known to interfere with some cartridges, in particular Grado cartridges, as I recall. But I could be wrong. Checking out the role of the motor in this phenomenon is actually easier than checking tonearm alignment.

Greg, the saving grace of Stevenson alignment is low distortion at inner grooves, compared to the two popular alternatives. One could argue about alignment all day but I strongly doubt that misalignment or Stevenson alignment can account for this problem. I’m now wondering whether motor is interacting with the cartridge as the two get closer to each other. To test this, with the platter spinning, pick up the tonearm and holding it above the platter but close as possible to it bring it over toward the spindle with volume control fully advanced.

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?

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.