What a great old Omega and backstory!
So many holes (figuratively speaking). From what I see, a jeweler/watchmaker over worked the edge of that caseback while polishing. It could have been due to a ding sometime in the last 50 years that he couldn't "even out" and the polishing took away too much material. The repair attempts of the past were poorly done perhaps - but I don't buy the Ph story since he could be straining to explain his own butchery.
Either way, the latter story about not being able to source a new caseback could have been true for 2 reasons - the jeweler may not have an Omega parts account, or Omega stopped making casebacks for that model.
I do believe a restoration on the caseback could be done via laser welding but the expense might be large. Rik is a good nearby option - the Ridleys in Texas are another. But try getting all the model and reference numbers together locally and give Otto Frei a call. THey have a lot of old inventory and if you're lucky a NOS caseback is sitting on a shelf.