Unless MWM has a retcon I am unaware of Norris was the first ever archduke of Deneb. When Arrival Vengeance made it to the real Strephon he found it amusing that Norris had promoted himself and then ratified it... oops, just read whulorigan's post, and he makes the same point
The chief difference between classical royalty and Imperial nobility is that royalty relied on divine authority whereas Imperial nobility is purely temporal. The Imperial Emperor doesn't seek divine recognition of his/her status, and the various nobles entitled by the Emperor similarly rely on Imperial authority rather than divine blessing.
Referring to the Imperial family as royalty isn't strictly speaking correct, but it certainly fits since they are the highest ranking noble family.
ISTR that he was the defacto "archduke" due to the fact that the Marches, in spite of being sandwiched between the Zhos, Vargr space and Swordys, was in essence one of the most stable regions of space during the rebellion. But no, I don't recall an official declaration of his ascension to being an archduke, he was just the one guy the Marches (and the larger Deneb area) could recognize. The Marches could rally about him, but the larger domain probably just nodded toward him because he had hard "rubber hits the road" military and administrative experience.
As for Margaret, to me she kind of came out of left field, and I never really understood the writeup for her history. When MT was published I was deep in finishing off my degree and going full boar in a career, so even though I was buying and perusing Traveller, a lot of what I read kind of got lost in my wet-ware's file system. Ergo when I read the Rebellion sourcebook … in spite of the setting established by all of the books, I just wasn't able to wrap my head around where she came from in a larger sense.
The Rebellion sourcebook felt like a post doctorate analysis for a game aimed at pre-teens and post college grads. Even though it was easy to read, I didn't get how the rebellion really stirred, and why if there were nobles, there wasn't more order. Hence the reason I was curious about archdukes (official or self declared) were slugging it out. But I guess it falls back on Lucan or Dulinor as the two usual suspects.