I hate French, at school I DELIBERTLY tried NOT to learn it(then again, fluent in German, Greek, Old and 'Vulgar' Latin, know a lot of Chinese(Mandrin), etc).

So you first of, 'et al', is a shortened ersion of 'et alli', and means literally 'and all', many people think it means 'and evrything', but this would be et 'et omni(a)'.

As for 'Coup de grace', to my best knowledge on this repulsive language, it means 'moment of mercy', or 'act of mercy', as with many languages it is not easy to translate directly. As a lecturer, I would ignore any Frenh refernces(as a matter of fact, according to marking criteria, am supposed to 'ignore' any 'quotes' in French, Latin 'et al'!).
Good luck, one piece of advice is in your essays please try and NOT use such expressions as they will not win you marks. It would be to your benefit to use the vernacular(native) language eligantly rather then these expressions. Latin, as much as I love, is really rarley used in ANY marking criteria.

Good luck!