I agree that it really is a little humor to convey the message that Lizzie should take care. It is not a nasty teasing, especially to Lizzie who tends to understand him better than the rest of the household.

Why would Mrs G. refer to Mr B. and not Mrs. B.. Hmmmmmmm Maybe because Mrs. G. understands Lizzie's feelings. I always thought that Jane and Lizzie received a bit of their education from Mrs G. as they would stay with the G's at times and they always wrote letters back and forth. I believe that the reason for the difference in the eldest was from Mr Bennet and the Gardiners as nothing could be learned from Mrs Bennet and Mrs Phillips.