Murphy has Lymphangiectasia.
It IS a serious disease and can be fatal, but the internist my vet consulted was fairly positive about being able to contain it - particularly in a Yorkie. She was of the opinion that Yorkies tend to fare better with it than some other breeds - I don't know whether it is true or not, but I like the positive thinking. Some articles about this condition are quite grim about the prognosis, but Murphy has not experienced too many symptoms, has maintained his weight, (give or take a few ounces), and appears to be responding to treatment, so we will just keep our fingers crossed that we can keep this under control. Right now, he's back to being his chipper little bossy self, eating well, sleeping well, and the runny poop seems to be improving.
In Googling the topic, I found a blog by the owner of a Chinese Crested who has had a hell of a time with this: Save Louie.
Right now I'm oddly grateful that Murphy's illness resulted in a granuloma that was removed surgically - if not for that growth and the pathologist's diagnosis, we might have wasted many more months of educated guesses. So while this is by no means a "happy" diagnosis, it is at least something we can try to live with - and so far, so good.