Is it permissible for a husband to address his wife with endearments such as "my darling" and the like during Hajj, and if not, what are the consequences of that?

There is no harm in saying "I miss you" to a mahram, as it is not the rafath (obscene speech) that is prohibited. However, it is preferable to keep the Hajj free from such emotional words, lest they lead to the forbidden rafath. Imam An-Nawawi mentioned that a pilgrim should keep his state of ihram free from insults, indecent talk, and addressing women with words related to intercourse, kissing, and similar forms of enjoyment.