would have done应该不仅仅指虚拟语气吧,是否应该还有推测的含义如Born in better time,he would have done credit to society,这句如用虚拟语气,来解释与现在情况相反,似乎不通再如she said that we would have finished the work by the end of this month最后We would if we were taught to put the fish back when we were young.For we would have learned the truth

并不是所有would have done 都是虚拟语气。在以下的结构中：条件从句had+动词过去分词，主句would have+动词过去分词，这时是虚拟语气，表示与过去发生的事实相违背。

若是推测have后面应加动词原形do而不是分词done

would have done有两种用法你都有提到：
1.虚拟语气：第一句 If born in better time,he...说明他“生不逢时”与
事实相反（最好前面加if)
第三句 We would if...说明“我们年轻时没有人教我们把鱼放回去”我们也
没有“学到这条真理”与事实相反.
2.在直接引语中的将来完成时改成过去时态的间接引语：你的第二句便是,
它的直接引语是
She said,"we will have finished the work by the end of this
month."

虚拟语气就是指与现实不符的一种情况，你说的例子我的理解是“如果他出生在一个好时代，也许他会对社会信任些。”用到的仍然是虚拟语气。