looking for advice to mitigate inner groove distortion

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?

cleeds, I used an alignment protractor and my eyeballs :)  I used a tracking force gauge to calibrate my tracking force.  The anti-skate on the P3 is pretty primitive so that part is imprecise,

lewm, both channels and the motor is located under the platter in the center and on the rear side of the plinth

adam8179 

I used an alignment protractor and my eyeballs ...

Not all cartridge alignment contractors are equally precise.  Many align the cartridge itself, which isn't ideal. The best alignment gauges, imo, align the cantilever. They're much trickier to use, but I think worth the effort.

It sounds like you’re using one of the freebie alignment printouts?
Some of those are okay for alignment only after having set overhang.
How did you set overhang?

Overhang must be set accurately first. Otherwise if overhang is off it will cause your cartridge to be rotated too much one way or the other, which you may not notice until the inner grooves where it becomes more obvious.

My favorite for being fast, easy and accurate, is the MoFi Geo-Disc  https://www.mofi.com/product-p/mfslgd.htm which you will notice has a ridge line running A to B which must be aimed directly at the tone arm spindle vertical axis. Then you tape the platter so it doesn’t move while you’re tweaking alignment.

Even though you are not using the Geo-Disc nothing changes. The Stevenson you’re using must still be placed accurately in relation to the tone arm spindle, and both the gauge and the platter must be held stationary.

Try again with these tips and overhang set first and see how it goes.