"Am convinced after three seasons of use that the Sawyer cartridges cannot be completely dried. Even after weeks sitting in a sunny window I can still get water drops out of them."
I agree and someone awhile back actually cut one open and confirmed same. Was that you?
"My takeaway is it's very important to do the chlorine solution backwash before storing for the winter, then at the season's start, backflush before the first trip."
I also agree with this and it is what I've done in the past. The reduced (almost no) flow problem occurs when I remove it from storage and am preparing it for that first trip. That's when I have to backflush, forward flush, soak in vinegar, etc. Takes about a half hour of fiddling to get it flowing correctly again.
"Some combination of bacteria and/or mildew is at work during that off period, clogging the works."
Maybe so. I don't really know the cause. Others have guessed it is the bleach or Seattle water.
"As to how visibly clear water clogging the filter in the field, it's likely to be bacterial biofilm. Have found field-backwashing can restore flow, even though it's a pita to do."
Haven't had a flow problem in the field yet and I've used it for up to a week at a time.
So far this year my pressure nozzle backflush (see link in post above) before and after each trip has kept things flowing nicely. Won't be putting it into storage again until Oct so we'll see what happens next spring.