Hi! I was also really curious about those! So I looked it up. I'll sum up what I found:
1. Principality. So basically this is a monarchical feudatory lead by a "Prince" as the title. In this manwha they called him King with having the church having higher sovereignty over the empire. Also, when we say feudatory, these are like owning a land like a landlord to it's tenants but just make it a King and his people i.e., the people paying taxes to the king. So since the principality Ditrian is managing is under the empire, all the belongings of the principality goes to the empire, making the Saintess the feudal lord.
2. About the concubine, I believe in korea, they call the ladies living inside the court (meaning living inside the palace) having ranks as concubines, i.e., Royal Concubine. Since the Saintess is the highest reigning woman in the empire, the woman married to the king of principality is called a concubine.
I may lack some of the info's.
I'm just here to read so, I'm fine if someone wanna give extra effort. SFX is optional.
Hahaha sorry I can't get over with Leticia getting translated as concubine. Shouldn't she be called consort instead. I also just realized what principality means but Ditrian is a king, right? So she could be called a queen? Ugh this royal terms...