As no experts chimed in, I'll share my opinion on my previous questions in hope it might help someone in the future. Keep in mind I'm very new to these issues, so I could be completely wrong!
After having handled several hundred examples of the AM Post issues and doing a bit of research, I feel like I have a better understanding of the paper varieties. The BEP was under a great deal of pressure to produce these stamps under very difficult conditions near the end of WW2. I believe they simply used whatever paper and gum they had on hand for the task. This resulted in significant variation in both paper and gum. It also resulted in a higher than usual number of plate flaws and perforation oddities.
A good study example is the 5Pf green which only exists in y and z papers. The y paper is significantly thinner (80-90 microns vs 100-120 microns) than the z paper, and thus easy to recognize with a micrometer. The z papers have visible wood pulp; however, the quantity and size of the pulp inclusions vary considerably -- even over a single printed sheet! Below is a sequence of the same stamp in z paper (click on the image for a high resolution version):
I placed the two with greatest difference next to each other. Below is a close-up (again, click on the image to get a larger version):
I am pretty sure these are all z paper. Even the one lightest in color shows clear wood pulp inclusions when observed backlit under magnification, and they all measure greater than 110 microns in thickness.
So the answer to my first question is that the "white" and "tan" papers described as "thick porous" are both z type papers -- just on the extreme ends of color variation. Some research shows that the album pages I have with the "white" & "tan" varieties are early versions of the "Bush" AMG album pages. Later versions of the album pages remove these varieties -- I suspect in recognition that these were simply shades of the same thing and thus impossible to incontestably differentiate given the vulgarities of time and inevitable paper toning.
As for references, I have found three books very useful. The Bush AMG one is in loose leaf form, and is *very* difficult to find. The APRL has a few older copies. The blue book is probably the most useful of the three; however, it is hard to find in the US (it is also in German).