Jesse was kind enough to meet with a few of us last night over dinner to explain some details about Acarthian noble traditions.
Nobles of all ranks are welcome to marry commoners, and culturally, this happens all the time in Acarthia. Commoners who marry nobles are given the honorary title of Lord or Lady. They do not wear white belts. They do not gain the power and privelege of the nobles who earned title personally. Honorary nobles are not bound to follow the Code of Chivalry. Children of nobles are given these honorary titles of Lord/Lady. Essentially, all a person gets out of that is slightly higher social standing -- really a better seat at the feast table, or good connections to get better box seats at the Acarthian Opera.
The honorary title is always Lord/Lady regardless of the rank of the noble spouse. Marry a knight? The spouse is Lord or Lady. Marry a baron? Lord/Lady. Marry the Duchess? The same.
Those persons who have passed the Tests of Chivalry, and are therefore eligible to hold title on their own may retain their lands and title even in marriage. For example, Baroness Elavir is engaged to marry Sir Eldred. Upon marriage, he can remain a knight, retain his lands, and still be Sir Eldred -- He does not automatically get "downgraded" to Lord Eldred.
If the spouse of a noble has passed the Tests of Chivalry, and is eligible to hold title on their own, the noble can petition their liege for an elevation of the spouse's title. Again using the above example, Baroness Elavir can petition Duchess Tiberion to bestow the title of Baron on Sir Eldred upon their marriage. There is no cultural norm for this, or expectation that it will or will not happen. There is precedent both for these elevations historically in Acarthia, and also lots of nobles whose spouses just took the honorary title of Lord/Lady and called it a day. It's entirely an in-play matter, and depends on the persons and the circumstance.
I hope I captured this accurately from the discussion with Jesse (and that he'll correct me if not), and that this helps shed some light on the initial questions.
Trace Moriarty