Re: 3rd year service - raging!
Sounds like an over-tightened drain plug to me!
My Alfa Main agent says this not uncommon. There are also past threads on this subject.
The sump is not weak or lacking in material around the sump threads but if the sump plug is done up with a 1/2" wrench, or even 3/8" come to that, then there is the possibility of damage to the threads.
I think the threads strip, rather than crack the sump. The answer is to use a 1/4" socket handle, then it can be tightened as much as you like.
Tricky situation with the garage. If it was no problem before going in to them but now is, it would seem to be their problem. You could mention to them about taking the car elsewhere for a second opinion, if they don't play ball.
It may of course be that the car has been driven over a speed bump, which initially damaged the sump, the oil change just finishing it off! Physical damage would be obvious, in this case.
Hope you get it sorted soon.