Technically, when Guen's father dies, she will inherit Cornwall and be Arthur's vassal (as Cornwall is now subject to the Pendragon crown) in addition to being his queen-consort.
Yup, and interestingly they were men's fashion before they were women's, unless I've been misled.
Yes. Lobotomies not necessary (with apologies to those who would like them).
It's definitely something I have in mind as a possible "retirement plan" for Guen if she doesn't want, or isn't able, to remain Queen of Britain.
So this is a departure from the main Arthurian tradition, but in the Guenverse, Cornwall isn't a kingdom so much as an independent territory, and Guen's father is not a king (nor is she a princess). Her family rose to prominence in Cornwall through many generations of strong leadership, and the Guenverse version of Cornwall has always vehemently claimed independence from the rest of Britain. While Leodegrance (Guen's father) still has a strong independent streak, he's not stupid -- he's noticed that the Vikings, the Saxons, and the Romans would all very much love to invade the whole island, and he can fend off one or two armies, but not three. Hence the alliance with Arthur.