That makes sense with regard to the data that is on the K-1 being sufficient for the S/H to perform his /her own calculations/analysis of gain.  With that being the case, the new 7203 seems a bit unusual because the so-called 'Reportable Gain' on line 34 is theoretical at best (it assumes, for example, that there are none of the adjustments that you proposed--such as S/H B inheriting from S/H A).  Numerous types of modifications could affect that number at the individual S/H level.

Good points.  Thank you for your comments on this.