Hi Howard!
Apologies that I've taken this long to get back to you! Thank you very much for your high praise; I'd love detailed feedback.
No worries. Of course I'd like feedback from you on my book, if you have time to read it. The nice thing about effectively self-publishing (e.g. with lulu as publisher) is that one can constantly revise.
Ok, here are the comments I made while reading, from typographical to stylistic, making a mythical twelve in total ;-)
1. I wouldn't call Dumnonia an "outcrop". Why not a peninsula?
2. You seem to have something missing in " whereupon ??? took up the names
Marcus Aurelius"
3. You imply Julian, son of Constantine, was left in Britain and took part in the British revolt. Actually he accompanied his father to Gaul and was murdered with him in September 411 while under arrest [Sozomen]
4. Your timeline is a bit unlikely, though certainly not impossible, in having Artorius Constantine born c.405 at the latest, but his grandson Arturius still vigorously fighting in c.537, at least 132 years later. (Mind you, my timeline is only a little less stretched out in this regard).
5. I like your idea of someone disfigured by fire claiming to be Vortigern as a way of reconciling Nennius' accounts.
6. Very frequently (maybe ubiquitously) you have Castum where you mean Castrum.
7. I guess you do know, from your careful wording, that tributary in reference to a river is a recent borrowing from Latin (by the analogy you mention).
8. I thought the tale of C and O really stretched credibility in terms of supposedly being part of an account by a Christian monk. The actual work is a magical light-hearted romp of course, and no one could imagine it to be true even if it were a tale told in the 6th century.
9. No Gwydre map Arthur?
10. In the Roman War episode, why is Lucius Tiberius sent to Armorica? If the aim is to force Arthur to relinquish the relics, why not to Britain?
11. When you have the Armoricans fighting with Arthur to liberate Armorica from the yoke of the Franks (Clothar?) and the Romans (Lucius Tiberius) are these Armoricans who have been expelled back to Britain? Or a fifth column? Or are parts of Armorica not in fact under this yoke?
12. In a footnote you say "Geoffrey has Gawain dying on the shores of Southampton." Actually it is Richborough.
If you want more feedback, I think you would get a lot more if you posted your work in the King Arthur facebook page. Serialised like here, so that each bit ends in footnotes would be well received I think. Just a suggestion.
In many ways, your work is what I aspire to, except on a considerably more ambitious scale. I've been focusing on the "history" which could have been absorbed by "later writers" and transformed into the heroic legend that we all know and love.
Thanks. I've toyed with the idea of writing a version of my book that could have been written in the late 6th century (the same time as your Paul Aurelian, but I have a different author in mind). But I'm torn by this problem: if such an account survived how did it get so distorted by later writers, and how come some parts ended up in some chronicles and others parts in other chronicles. Consequently I've just gone with a synthesis that is quite, but not exactly, like a medieval chronicle.
I've been toying with the prospect of turning this into an illustrated mini-publication and even have an artist in mind, but I might have to find more disposable income before entertaining that notion.

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Yes, I gather that can make a big difference, even just in terms of cover art. I'm working now on a condensed e-book (kindle) version of my work, and wondering about professional cover art for that.
It is indeed more of a pseudo-history than alternate history, but it's very much square peg, round hole - and this board gets more attention than the more fantastical "Alien Space Bats" subforum, which is flooded with A Song of Ice and Fire fan fiction anyway (kidding, I love you guys.)
Well I make a distinction between pseudohistory, such as that of Geoffrey of Monmouth i.e. "rightly described as ... false history, because of its anachronisms, hyperbole, and blatant contradictions of more reliable chronicles." and quasihistory: having "the appearance of a history, and not actually falsifiable." (quotes from my Preface). [Is there really an alien space bats subforum?] I know I'm splitting hairs, but I wonder about an alternate historiography genre --- where the history itself is a quashistory, but the historiography is different from our world. So then one would not have to worry about the issues I mentioned above i.e. if such an account survived how did it get so distorted by later writers, and how come some parts ended up in some chronicles and others parts in other chronicles.
Best,
Howard