Ok, this quite obviously has nothing to do with port forwarding, firewalls and the like. After some further testing it became obvious to me that:
SA will show but not share files under UNC pathnames.
I can confirm this because I put a couple of the files in my collection to a local drive on machine A (e:\temp) and I was able to d/l from machine B immediately. In other words, you shouldn't add files to the SA library over
My Network Places - these files will show up on the p2p network but will not be downloadable, for some reason I can't explain.
Yes, mapping the network drive works, but it's not an elegant solution for my purposes. Besides, SA should plainly refuse UNC pathnames if it doesn't support them. The way it works now is illogical and confusing. I personally consider it a bug. Le me see if there's a bug reporting page somewhere...
PS I'm running SA 2.5.2.0 32-bit on Windows XP Pro SP3