mercurydancer,
A defence would be how many repairs are made in similar circumstances using titanium. If the mechanic was one of many who used such materials then its the company who would be equally culpable, but if he alone decided to repair using the material and it never happened before, or since, then its a frolic of his own.
I have never encountered "a frolic of his own" as a concept of French justice.
Why are you and others trying to re-try the case?
It has been heard.
Findings were made and sentences given.
The various parties judged responsible were sanctioned. It is a little bit late to speak of potential defences, even if they were defences which held some sway under French law.
Why do so many contributors want to believe that their local laws, jurisprudence and standards apply somewhere else?