I don't think you are over-thinking the romantic element between Jing er and the Empress at all. Jing er is referenced by other characters as, in translation, "the Empress' favorite" and the "Empress' bitch." I don't know Mandarin well enough to know what connotations the words they were translated from have. Also, in the almost sex scene with Dee, he tells Jing er not to force herself to have sex with him. Their dialogue seems to have different layers of meaning, but one I got from it was that she may not be inclined to have sex with a man. He also mentions that although she may feel she was required to sleep with him that doing so would actually make the Empress jealous and suspicious of Jing er - which I read as a hint to their relationship.

The first time we see the Empress and Jing er together they are in full armor (Jing er's first act is to protect the Empress) and throughout, except in the "private" dinner scene with the Empress and when she dies Jing er is pretty butch and in the historical context androgynous.

Can't just be my interpretation either because the film's creators chose to dress her in masculine fashion (except in the dinner scene w/the Empress in her inner chambers and when she is set to seduce Dee). The movie also has a very strong feminist theme.

The Empress says repeatedly that what she aspires to is unprecedented for a woman and that the obstacles she faces are because of sex discrimination. Her right hand "man" is a woman and given the setting it would be unlikely that she could have an intimate relationship with a man - her choices for love and sharing her bed (though the film doesn't go there) would have to be female.

At the end Jing er chooses to be sent back to die in the Empress' arms. If she were just a lackey who failed she might have been afraid to go back, but their relationship is deeper than that.