整个句子的主干是It goes without saying that...(换句话说...)
that后面的句子是一个虚拟语气(与将来事实相反),后面这个部分省略了if,正常的应该是if her enemies should discover...
D'Artagnan's impatience to return toward Paris是主语(其中的动词不定式部分是定语修饰impatience),had for a cause the danger是谓语,which部分是定语修饰danger.