I've always thought that them addressing Edward by his first name was through the family connection they had. When they speak of Edward to other people (like over at Barton Park), they still call him Mr. Ferrars.

In Emma, Mr. Knightley calls Emma 'Miss Woodhouse' in company, but by her first name when they are in private or less formal situations and there is a similar family connection there (siblings married to each other). She, however, doesn't want to call him 'George', even after they are engaged. I've always thought it was due to their age difference, the respect she felt for him, and because that's just how she thought of him in her heart.

Ch. 53

'Mr. Knightley.' You always called me, 'Mr. Knightley;' and, from habit, it has not so very formal a sound. And yet it is formal. I want you to call me something else, but I do not know what."

"I remember once calling you 'George,' in one of my amiable fits, about ten years ago. I did it because I thought it would offend you; but, as you made no objection, I never did it again."

"And cannot you call me 'George' now?"

"Impossible! I never can call you any thing but 'Mr. Knightley.'