To Elizabeth it appeared that, had her family made an agreement to expose themselves as much as they could during the evening, it would have been impossible for them to play their parts with more spirit, or finer success; and happy did she think it for Bingley and her sister that some of the exhibition had escaped his notice, and that his feelings were not of a sort to be much distressed by the folly which he must have witnessed.

这是有分号加and连接的一个并列句。

第一分句为 To Elizabeth...or finer success; 第二分句为and happy...must have witnessed.

第一分句为it appeared that句型。that从句本身又是一个虚拟条件句，had her family made an agreement... = if her family had made an agreement...，所以 it would have been impossible...为虚拟语气，表示与过去事实相反的推测。

第二分句 happy did she think it for Bingley and her sister that...and that...

happy在此为lucky的意思，作宾补，it为形式宾语，外置的二个that从句为真正宾语。宾补前置表示强调，由此导致部分倒装句的用法，语法书并未提及。但《傲慢与偏见》的作者既然这么用了，我们也只好接受。至少倒装让我们不会误解happy是描述主语she的状态，而能够理解happy（lucky）是宾补，是主语she对二个that从句的看法。