
You are very right on this based on the fact that the setting this manga is in. In fact, it is a realistic portrayal of ancient monarchy setting, which we all know that the emperor wouldn't have just 1 wife. The other shoujo manga whereby the emperor has only 1 wife just wouldn't happen in real history...
<spoiler> Yes, he slept with the other concubines in order to have heirs. He doesn't have feelings for them. He only has feelings for Kan. </spoiler>

Just realise the spoiler tag doesn't work here... will be more careful next time... Sorry!
I wonder if the emperor likes her or not cause I didn't see any inkling of that. More like he wanted to protect her but like...how is she supposed to get promoted(in her career) if she is now a concubine albeit the empress. The train of thought behind making her a concubine has passed me by. Can someone explain this to me.