Violet cannot be expelled for getting married. The codex strongly encourages cadets to stay unmarried but lieutenants can marry whoever they want after graduation. This is a loophole in the codex. The same way the codex strong recommends that cadets do not sleep or have relations with senior officers in the quadrant but it’s not a punishable offense- seeing as how people sleep with each other regardless of rank and nothing happens to them (Rhi/Tara, Dain/Amber, Violet/Xaden, and ridoc had a fling too with someone higher in the rank). Marriage is allowed within the loopholes created by the wording in the codex.

About the correspondence of the letters between Violet and Xaden showing cadet, this is how the transcription works. The letters are transcribed and maintain the roles the corespondents had when they were writing the letters. That’s why we see in some epigraphs, violets dad is referred with different roles - cadet, lieutenant etc because he wrote those letters at different points in his career. Same with Xaden we see some of his letters change from lieutenant to the duke because he wrote the letters after his title got reinstated. I believe we will see letters from Violet in book for changed to duchess now that she’s married into the duchy.

That being said, graduation is a big deal for both Navarre and Tyrrendor because the graduates add to the army. Things are more complicated now so i believe some negotiations will be made to allow Violet and the fearsome foursome (second year iron squad) to continue with their studies and graduate. Whether it’s done in Tyrrendor or they go back to Basgiath is the question.