[X] Be friendly, show her around and introduce her to your spirits. They will be members of the party too.
Finding out how our military adjutant reacts when first meeting with a group of people who do not conform to imperial standards of formality is essential to mission planning. So is getting her used to talking with Hanyi, who is going to be a big part of the diplomatic effort. These are the diplomatic mission parameters, and if they are going to cause a conflict, it will be best to find out and work on that from the outset rather than having her and Hanyi get into an argument while we're half way to barbarian country.
Yes, going informal will probably be off-putting for Xia Lin, but the worse case scenario there is that she is dismayed at our lack of manners and decides that she is more comfortable working with the considerably more formal Cai Renxiang. That is hardly a disaster, I think.