Overall, a good read. A few niggles which you could improve on, however...
"Good Goddess, Lance, go away!"
The Britons were Christians by that point. King Arthur's court would have been especially devout, particularly in the Geoffrey of Monmouth versions. Indeed, even the prophetic Merlin was Christian, - despite the portrayal of him as a druidic pagan by some later authors, who took severe liberties.
This "Goddess" nonsense features later in your story, at several points, as well... Arthur, in particular, fought adamantly against such paganism. It's positive inclusion is downright strange.
"He's pardoning them all. Probably right this minute."
That's... a new take. Not really the sort of man Arthur was, from what I've seen.
"The wedding takes place in the Great Henge. People say that Merlin built the Henge long ago. Certainly it must have taken a great sorcerer, a brilliant engineer, or an army of giants to raise the massive stones."
A nice touch.
As an aside, in the account given by ol' Galfridus the stones were brought over from Ireland... And, as it turns out, they really, historically, were, - and from the same area described in the story. Just one of those interesting little snippets that questions just where the 'pseudo' comes into the pseudohistory. Same problem we have with Herodotus, really, albeit with far less literary flare.
Although it would not be called the "Great Henge," because that word is a Saxon development. Geoffrey called the site the "Mount of Ambrius."
"Both have their hair cut short and wear formal tunics and leggings rather than dresses."
Careful now. You can get burned at the stake for that.
"the small but ancient Order of Boudica."
Oh no no no. That makes no sense whatsoever. Boudicca was absolutely detested by the Roman historians, - who give us the only account anybody has of the woman. To suggest she would be venerated in a knightly order is absurd enough, but an order of chivalry?! This was a mass-murdering, city-burning dishonourable maniac who sliced women's breasts off so they could sew them into their mouths!
If I might be so bold, perhaps the Order of Gwendolen would be more appropriate, for a female military leader, who features within the Arthurian tradition... sort of.
Also, this tale references fae at numerous points. This should be rather striking, for the Anglo Saxons (who really did most of the work on the fae, in Britain) found them to be terrifying, whimsical psychopaths. I would imagine describing someone as having a "white beard and glint of fae" to be more than a little off-putting.
"It's better that way. Can you imagine if they'd made me marry some fourteen-year-old? This is weird enough already."
That sentiment is rather anachronistic. I don't even think it is properly endorsed in the present day.
"Merlin must use this space for observing the stars; I see an armillary sphere, a celestial globe, and a couple of strange metal tubes whose purpose I don't know."
I should never be reminded of Orgone-fuelled Cloudbusters in an Arthurian story!
"her husband Hrothulf's Romano-Saxon troops would never fight as loyally for our king as would native British troops"
Remember, at this point, Arthur would be ruling over a somewhat Romano-British land. Whilst Rome did exist as an independent entity to the Kingdom of Britain, the connection between the Roman Emperors and the Kings of Britain was not forgotten; Arthur was succeeded by a Constantine, after all.
"Lancelot's armor is in the Gaulish style, but he wears the sword-and-lily crest of Avalon."
A little peculiar. Wouldn't the heraldry of Avalon be to do with Apples, not the more Frankish development of the lily (originally the toad)?
"In my ideal world, Guen, there is no king, because the world doesn't need a king. Or a queen, either, I suppose..."