I heard that Chabad instituted a requirement for shluchim to be married because Zalman Schachter-Shalomi went off the derech. Is this historically accurate, and are Chabad shluchim required to be married nowadays?
I'm not going to comment on your example b/c I don't know enough about him. However, yes, shluchim must be married, or when they are single they never go out somewhere by themselves. Shlichus must not be done alone. Not only this, but a big aspect of the success that shulchim have is be inviting people to their house to give their guests a glimpse of the beauty of a chassidishe family who are happy to keep Mitzvas and be close with the Rebbe and how chassidus permeates everything they do with a refinement not often seen in the secular world.