A profile like yours is a decent basis for follow-on observations. That doesn't mean it's right, or that it's perfect, it's just an attempt to model the known reports and facts in a consistent narrative so as to suggest where further investigations may be more fruitful. It doesn't deny "something completely different" as a possibly.
I have two points related to the profile. One is a hunch is that the terrorist cell had one and only one vial of the anthrax and they were rushed to use it for some reason -- panic maybe. Why then would the letters contain different granularities? As another freeper suggested it could be because the very fine stuff settles to the bottom of the vial. As the powder was carefully spooned out of the vial early letters got a coarse grain, later letters a fine grain. If so, no connection to a biological lab is needed to be infered.
The second is the 'G' and '6' as handwritten on the letters and envelopes are peculiar and perhaps indicate a arabic cursive normal usage.
I'm anxious to see (if we are allowed to see it) the lab analysis of each specimen from Ft. Detrick. I'm particularly curious if the only difference between envelopes is size of the spores - or if the batches were processed differently!