and when lastly,upon his insisting in a positive and angry manner to see them,they came to greet him,whom should he see in their company,but the hated Goneril,who had come to tell her own story and set her sister against the king her father!

我简化一下这个句子：

When they came to meet him, whom should he see in their company but Coneril !(?).

当他们来见他的时候，他看到和他们在一起的除了Goneril还能有谁！

这个主句是个由修辞性问句演变而来的广义的感叹句。即和他们一起来的竟然是Coneril！

but在此为介词，意为除了。

特殊疑问句也可以用作修辞性问句。肯定的修辞性表示强烈的否定意义，否定的修辞性问句表示强烈的肯定意义。本句在形式上是肯定句，所以表示的是强烈的否定意义。即除了Coneril, 和他们一起来的没有别人。

修辞性问句句末读骤降调，就成了广义的感叹句。例如：

Who doesn't know Ma Yun? 谁不知道马云？

= Everyone knows Ma Yun. 所有人都认识马云。

Who doesn't know Ma Yun! 谁不知道马云！

= Everyone knows Ma Yun! 所有人都知道马云！