Dan, i had a read back through your thread and when your mechanic friend left the allen key in the belt, you found pieces of pulley in the bottom...
At the time you weren't sure what they were - did you ever change that lower crank pulley - i only ask as when i was removing mine (original one so heating it up / hammering the bolt the pulleys inside edge broke off into pieces like your pics (page 74 ish if i remember right)
Those pics were 100% pieces of the lower crank pulley.
Because of the way the pulleys break (there undercut) they loose about 5mm - 10mm of the width of the pulley but they break accurately just leaving a narrower pulley.... to the untrained eye (or if the engines in it's very hard to see) the pulley might look ok but if it's too narrow then bad times..... if you didn't change it maybe the belt was running on and off the edge which can cause a knick then snap.
Just thinking out loud trying to piece together why this might of happened....
Good on you to get cracking and not sink into despair bud.
Personally though I would try and rule out things to stop it from happening again i.e maybe take the OEM cam belt tensioner outta the equation by fitting a manual Integrated Engineering one on & New idler pulley on, check the water pump is nice and free no play in the bearings & New lower pulley (if you didn't already).
Good luck bud.