But I simply couldn’t find any reference anywhere at all that the fresh air breather was anything but a fresh air supply to the crankcase, and that blow-by exiting this port into the inlet is a fault or failsafe mode, according to the design intent of stock pcv systems.
What have you found? Just a description as a "breather" and a non-exhaustive description that air enters through there?
The valve cover "breathers" probably draw in air under vacuum conditions (because the greater manifold vacuum is drawing crank gas through the PCV) but surely under boost conditions both the crank breather and valve cover breathers are expelling gas due to the ported vacuum from the pre-turbo pipe. EDIT - the simple diagram you've pasted above seems to confirm the "direction of gas depends" matter.
If my brand-new engine* collects fluid in the valve cover "breathers" catch can AND having a catch can in that loop is a common Subaru aftermarket thing, surely this brings an end to the hypothesis that gas only ever flows in one direction through the valve cover "breathers".
[* albeit with exaggerated E85 crank water vapour, some enlarged breather passages and two catch cans]
Agreed it would be interesting to get more of a handle on different routing, restrictions, extreme crank vacuum methods etc. For my car I'm happy with the factory loops with catch cans added (and some flow enlargement). My further experimentation will probably be limited to "better gas scrubbing" within the same basic architecture for an even cleaner intercooler.