Wow, lots of insight here, and thanks everyone for the concerns!
The car is currently 50 miles away as I had to get back home to work the same evening. I'll properly inspect the other side when I get it back home to my preferred shop, I guess that should clear up whether the retaining washer was left out, ball bearings were used or the nut was over-tightened.
@blown motor
A wrongly tightened nut sounds plausible to me, and that's what the mechanic's first instinct was as well. I put a little over 2,000 miles on the car in 2 years, not sure how many mile the previous owner put on after putting in disk brakes.
@anlauto
I don't have any radio in, it was only the motor we were listening to! We drove down from Maine over some curvy routes (route 3) and then onto I 93, but just smooth sailing. The brakes felt a little weird at a toll station a couple miles up as I remember now, then on the highway I think I told my buddy I heard something once or twice, before I definitely heard a clunk that made me start to pull over, right before the wheel decided to have its own way.
@AARuFAST
Boah, nice input with the fender. The mechanic gave me a blank stare when I asked him about fixing the fender, I hope he doesn't do more damage to it when bending it so the wheel will fit. For the alignment, I had new wheels and always a slight left-pull, so this sounded to be the perfect opportunity. However the guy suggested to replace the left-side pitman arm first (because it was loose), but said it's still okay to drive around, just something to do rather sooner than later. He called me today that the part had arrived...
@brads70
That should be the kit, exactly! Haven't recovered the rotor, only what's in the images.
I'll keep you guys posted, thanks for all the thoughts!