]I was thinking she found the news of her nephew marrying Miss Bennet alarming enough to set out at a late hour and impatient enough to insist on going on all night.

But don't you think that very dangerous, to travel at night? I think she has a house in London, even if she hasn't, she could have stayed at an inn. Btw, we don't know when she set out, do we? But that, as you say, we shall never know.