WOW!!! Just a Mechanic thinking here
Do you think they could have dyno'd it and over reved it during the process?
This could have caused semi seizure or piston ring hitting a cylinder top ridge breaking one or more rings maybe.
The blow bye would increase the crankcase pressure to the degree that the pressure just blew the cover gasket and pushed oil out.
I know its no comfort to you but it sounds like someone has stuffed up big time and stopped any further work just wanting to get the bike away from their premises maybe hoping to blame the owner for something... Bastards!
I would find some of ya big mates and go back..dont know nif I would let them try to fix it though, they may cover up some other problem..
Do you have the dyno slip?
It might tell a story if they gave you a true copy.
Sorry to here this mate.. bloody heartbreaking I would think...