Further to your question regarding differences between various alignments, may I refer you to page XV of my analysis on optimum tonearm alignment (2008), located at:http://www.vinylengine.com/phpBB2/viewtopic.php?t=4854
On that page is shown a graph, which shows the weighted tracking error (WTE) across the groove radii of 146.05 to 60.325 mm, for three alignment strategies. Tracking distortion is proportional to the WTE, as established by Löfgren in 1938.
Now, a closer look at the three plots on that graph may help explain the effects of the different alignment strategies, ie, different offset angle/overhang pairs. After all, the only differences behind the three plots is the differences in the offset angle and overhang values selected for each. Changes to those values will result in changes to the shape of the WTE plot. The null radii are located at those groove radii where the WTE is zero, ie, where the WTE plots cross the zero axis. Areas on the plots with the higher WTE values - note that positive and negative values are treated equally for distortion purposes - will produce the higher distortion at those groove radii, and vise versa.
The Löfgren A alignment provides the minimum overall distortion (ie, the three WTE peaks are minimised and equal), the Löfgren B alignment reduces somewhat the distortion between the null radii but increases the distortion at the inner groove area, while the Stevenson A alignment has small to zero distortion at the very inner groove area, but has the largest WTE (ie distortion) over most (around 75 percent) of the record playing surface.
Of further note is that the WTE for the three alignments over the outer one third of the groove radii (say 115 to 146 mm) are somewhat similar. However, the WTE for the three plots for the inner two-thirds of the record radii are quite different, resulting in different distortion results for that area of the record.
The three plots on page XV are three examples of alignment strategies, and the equations used to obtain the offset angle and overhang values for them are shown in my analysis, courtesy of their authors (Löfgren in 1938, and Stevenson in 1966).
Hope this assists.