I think "stress corrosion cracking" is a red herring here. While carbon steel certainly can corrode rapidly in salt water, the chemistry just is not the same as with stainless alloys that stress crack in the presence of chloride. If there was enough salt water bouncing around in the motor housing to seriously corrode the shaft, I would expect LOTS of other issues to go south first! This would be easy to confirm with a look at a sheared-off shaft. (Anybody have a photo?)
In normal operation the motor simply can not develop enough torque to shear off the shaft. I have certainly stalled the outhaul motor many times while flattening the sail without a mechanical failure. While it might be possible that there was an epidemic of bad steel bar stock for motor shafts, this seems very unlikely.
I have never done a post-mortem on one of these that has failed, so all this is a guess... There are two possibilities I see, both related to wear in the gear box.
A worn or broken bearing could allow enough runout of the worm screw that it imparts a significant side loading on the motor shaft. This could result in a failure of the kind seen. I think this is the most likely cause.
Another possibility is either by wear of the gear teeth, or a bearing failure, the worm suddenly jams in the gear while running at speed. I can imagine this kind of shock loading causing a shear failure. I don't see this happening by an external load, the 60:1 gear ratio would just not transmit enough of a shock load back to the motor--but you never know...
Again, all just SWAG (Scientific Wild Ass Guessing) without having a failed unit in hand.
It also appears incorrect to say that older boats had Bonfiglioli and new ones had Leroy-Somers drive trains (or vice versa). Amel seems to have switched back and forth during the production run. Our boat (#160) has Leroy-Somers, and I have seen higher hull numbers with either supplier.