Lucy doesn’t seem to be what she was.—No.----- so much in th

Having seen ...无法在这里被当成主语
Her seeing so much in the war 正确.
如果以 Having seen ...起句也能表达同样的意思,但句子必须改写为：
Having seen so much in the war,she is more thoughtful now.
如此,She 就成为句子的主语.