After being evaluated by a physical therapist yesterday afternoon and speaking to my doctor later that evening, I am happy to report that my hamstring tendon is intact!
This is good news! If there's no tear, it means I'm not in immediate danger of a major rupture - and that's huge. Unfortunately, the pain still remains. They're attributing that to some chronic and repetitive stress in a way that makes it sound a lot like a form of arthritis.
I've essentially got some very loose tendons and ligaments around my knees (probably from years and years of breaststroke) and flat feet, which combine to throw my alignment way, way off-kilter. The result is some unpleasant bone-on-bone grinding. The pain I'm suffering now is acute, but the issue I'm dealing with is probably going to be a problem for me for the rest of my life.
The plan of attack is as follows: a break from running, a mega anti-inflammatory and a handful of exercises intended to strengthen the stabilizing muscles (glutes, hamstrings, etc.) that are supposed to keep me tracking straight. I'll have to keep up (and expand upon) these strength exercises in the off-season, and that's where Kristine's yoga, coaching and injury-prevention recommendations will come into play.
I don't know if this means I'll be able to run in Zurich, or what kind of pain I'll be in at this point. I could be looking at a very long walk. But at least I'll (probably) finish.