I went back through the photos I took and found this one, in which you can kind of see the busted chain link: if you look just above the cassette, where the chain leaves the pulley, you can see it. Not sure if this really proves anything, ultimately, but the more I think about it, the more it seems that either the mechanic didn't insert the pin properly, or the chain was defective, because I don't think the chain could have broken like that from the impact of it being slammed into the cassette when the derailleur went.
