I wouldn't be surprised if that is Tantai Lan's specific role in the court, and why she has been hanging around the palace ever since the Emperor's wedding so much. Since she is a military officer, rather than a career courtier, she is somewhat shielded from courtly intrigue, making it easier for her to act under the Dowager's nose.
That and she's wooing her future partner. Let's not forget that!
I'm not sure if she is really "shielded" from the intrigue. Apparently, there is a frontier pass somewhere waiting for her, and if anything, a punitive relocation seems to be easier than having to stage an incident to get rid of whoever else.
I also don't understand why to pick Tantai Lan specifically. Sure, childhood friends, but you can't tell me that outside of her, Yu and Ming (who is still an option?) there are no other nobles around who would fit the role - especially since it seems Yu was also pulled out of thin air all of a sudden.