Here are the first three questions (sorry for the delay).

Q1: In C-2-7 the SIF is identified at the center of the header. More applicable data suggests that it should be applied at the branch/run-surface side too. Bending moments along the stub can vary in either direction. It should not make much difference except in very tight piping systems, and in this case, erring on the conservative side was considered reasonable.

Q2: The stress is defined using the SIF and the section modulus of the matching pipe and moment (more or less). When putting FEATools together we knew that we needed to compute the stress at the intersection point but needed a strong (rigid) element there too. Rigid elements in different pipe stress programs have different meanings. We could call it rigid (but would not get the stress), or we could use a short element which was rigid, adjust the SIF, and get the stress computed appropriately there.

Q3: See the answer for (2) above. The objective is to compute the appropriate stress as (SIF)(M/Z). For the rigid stub from the surface to the centerline an alternate Za is used, and so an alternate SIFa is needed so that SIFa / Za = SIF/Z.