2. What information does Defarge receive from Jacques? What seems familiar about this?

Jacques recalls the harrowing scene of a man hanging by a chain underneath the Marquis' carriage. This turns out to be an important image for the revolutionaries because it incites enough anger to let them forget about moral sensibilities regarding a possible "war crime." Although I'm not sure about your second question, I would venture to say that this is familiar precisely because it represents yet another example of the aristocracy's heinousness.

I'm pretty sure the answer to the last question is the marquis had encountered the man before in the streets when the marquis ran his child over with his carriage and the boy ended up dying.