Hmm, OK. Thanks. That's two good reasons not to do extensive filing (the first being the wheel itself spinning off-line). When I get back home I may put a couple of strokes onto the very opening itself to make sure there isn't just a bit of a "hump" in the opening (though not in the part where the axle actually gets seated), but failing a miraculous fix there I'll see what the instrumentation verdict is.
Ugh--hope I don't have to get the dropout replaced...