Claudio confirmed all this with saying that Hero had told him Beatrice was so in love with Benedick, that she would certainly die of grief, if he could not be brought to love her; which Leonato and Claudio seemed to agree was impossible, he having always been such a railer against all fair ladies, and in particular against Beatrice.

which为关系代词，引导非限制性定语从句。which的先行项为 be brought to love her.

这个定语从句前用的是分号，分号相当于并列连词，所以注释本认为这里which应改为that。但that并不是从属连词引导主语从句，而是指示代词，仍然指代be brought to love her 这个不带to的不定式。that为agree后宾语从句的主语。

其实这个句子应该将分号改为逗号，which引导非限制性定语从句是最简单的。

which不能改为what, 因为what引导名词性关系分句，相当于一个名词词组，改为what后，分号后面就没有主谓结构了。

值得注意的是，这个句子最后独立主格结构 he having always been...中，he为主格代词与分词完成体构成独立主格结构。这才是真正的独立主格。