Does she ever call Darcy by his first name? When she says, "Fitzwilliam" in Chapter 31, she is talking to Colonel Fitzwilliam.

In P&P, use of last name seems to be the way the gentlemen talk to/about each other, and the ladies talk that way about the gentlemen as well. But they might use first names and/or nicknames in private. We never hear intimate conversations between a husband and wife, so the novel doesn't give us any clues on this matter.