User blog comment:B214/New anime 2017 discussion blog/@comment-97.86.117.132-20161230234256/@comment-10330822-20161231181754

Well, if it says she "retired" the position, it means she was it before she retired.

What I was thinking with my second option is that; Issei and Rias got married, they became the heads of Gremory, but Issei was given a higher title. So, Issei established his own house, with a higher title, and Rias passed off the old title to Millicas. So, for example, if Issei was made an Archduke, Rias would retire as the Duchess Regent of Gremory and pass the title of regent to Millicas. Then, she would reign as the Archduchess Consort for Issei.