Ok I found the solution for the 404 :
I already had these lines in my config.xml :
It had a bad side effect : the whole controller was overriden, instead of just overloading the saveAction() function…
So I simply added these lines to my config.xml (just before the above lines), in order to “partially reverse” the overloading process :
Now it’s ok : only the saveAction() function is overloaded.