理解正确.
=Lord knows what the ruffian might have done if we had been alone on the stage for two days .
if虚拟条件句中若有had,were,should,可把这三个词提前,把if去掉,成为倒装句
如：
If I had seen him yesterday,I would have asked him to come.
=Had I seen him yesterday,I would have asked him to come.