I speak fluent Spanish, and while that's true in general, it has exceptions (such as "g" and "c"). I don't believe that that holds true for French, either, though I'm no Francophone.
Yes of course.
Though putting aside the crapload of exceptions that come from the old french, every diphtong remains the same and follow the same rules. As for m or n, if you take it in the case of "un" or "bonbon", they are not "n" and "m" anymore, but a new combination that create a whole new diphtong.
So well yes, I misread in the first place and thats not a matter of letter that do never change, but a matter of diphtongs and combinations. Which is still isn't the case in english, for example, where every pronounciation is totally independant of the spelling without apparent rules.