Bottom braket failure shortly after overhaul. Is this reasonable?

[Yellow Saddle]

That's a better explanation -

Except for the flying part, and exploded ball bearing.

 

[Ajax Bay]

Looks like the ball bearings broke open, too. I went back to the bike shop that did the work and they said that the bearings are inspected, and if they look fine they are wiped clean and reinserted. They were able to add new bearings . . .

Did you see the "ball bearings broke[n] open"? Just asking this because they don't, and any 'inspection' of the bearings can only be cursory and the idea of them [being taken out and] "wiped clean and reinserted" is misconstrued (at best). "Add[ing] new bearings"?? Nope.

the crank arm shouldn't have come off - not torqued by the shop.

The crank arm didn't 'come off' for 600-700 miles - and this crank gave way suddenly, with no sign of looseness beforehand. So we've not established whether it broke or 'came off', aiui. Let's not indulge in blame games till the facts are clear(er).