Does "perfect alignment of cart" eliminate 100% ?

It's called "un-distortion", a world where everything becomes crystal clear and transparent. They use it at airports.

None of the currently popular protractors use Stevenson, as far as I know, because Stevenson is slightly out of favor compared to Baerwald and Lofgren A, in that order of preference. You might google it to see if there is something available. There is a rudimentary protractor available on Vinyl Engine for free, but you have to print it at 1:1 ratio and mount it in some rigid substrate in order to use it accurately. Stevenson gives an inner null point at about 60mm, which is at the innermost of any inner grooves. I recently saw some measurements of typical LPs, and in many cases the grooves had run out before the 60mm radius from the spindle as centerpoint, i.e., the true inner r was >60mm. This means that in some cases Stevenson will not give you two null points over the course of an LP, only the outer one. Having said all that, if you have a vintage Japanese tonearm, chances are it was optimized for Stevenson (In that the offset angle of the headshell is best for Stevenson alignment), and you might therefore want to use it. Finally, don't get overly obsessed with this issue; it ain't worth it.