I think I can relate the stratum server's difficulty to the network difficulty, roughly.

By experimenting with the setting I found setting it at 1 and too many useless shares came in, and adjusting a bit higher, and there would have been maybe 20 to 50 shares submitted before finding a block.  (The network doesn't have a great amount of hashpower, there's only a few miners).  So that seemed to be a good setting.  It's low enough for all CPU miners to find shares and share the block reward, but not too high so that miners only find shares once in a while.  It seems to be mainly for filtering out low shares, and is set by the pool, and might be updated at a later time if needed.

So stratum difficulty is just set by pools and reflects the kinds of hardware and software used and how many miners there are, and is low enough so that all miners with adequate hashpower can share in blocks found.

All the different difficulty numbers were getting confusing, especially because they are all expressed a bit differently.  I can see it more clearly now, I just have to jump back and forth between difficulty numbers expressed in hex, as whole numbers and as floating point numbers.  I suppose with the numbers below zero it just means with difficulty_1_target / current_target - over time (unusually) the current target is higher than the difficulty_1_target maybe.

Thanks for your reply!