Frankly speaking, the central point in a flange calculation should be the "Assembly Bolt Load".
It is intuitive to feel that higher loads will be required depending on the internal pressure at operation. Re-tightening the bolts may be required after operations starts, but there is a warning...not re-tighten too often as this might affect the gasket and produce more leakage!

Surprisingly, this number (bolts load) does not affect the ASME VIII Div1 App 2 calculation as App2 is not mentioning such "user" bolt load. Why?

You may think that App2 (Taylor- Forge works) is a procedure focused on flange dimensioning case(and, in my opinion, we are forcing this reality by trying to adapt this procedure for flange checking case).

Anyway, ASME VIII Div1 appendix S is clear saying "in any event, it is evident that an initial bolt stress higher than the design value may and, in some cases, must be developed in the tightening operation, and is the intent of this Division that such a practice is permissible [...]"

I try to summarize my understanding:
- DIV1 App 2 is giving the minimum required bolts load Wm1, Wm2 and W, not the bolt actual loads. The allowable bolt stress is a low number just to overdimension the bolts.
- DIV1 App S gives the possibility (they say "may" and "in some cases must") to work with other bolts load, which is greater than max (Wm1, Wm2, W).