I enjoyed your Youtube video. I have watched it through three times now.
I like the background information you gave, and the maps and the depiction of the movement of armies. I like when you zoomed in on Tahou. I like the way you bring in the images of vordak, ogrons and drakkarim when they are mentioned, and the images of Joe Dever’s actual brief on what he wanted to be depicted on the cover.
The automated ‘audio to text’ subtitles are poor, however. Many of the proper nouns (such as locations or names) it does not at all recognise and comes up with nonsense. It seems to be suggesting that ‘Croatia’ is being invaded by armies led by by ‘war decks’ under the command of someone called variously dark lord ‘gnog’ or ‘gnaw’ - in a book written by ‘Judea’!
The location of this text gets in the way of interesting information. For example time 2:37 and 2:56 and 3:39 where it obscures the writing. It is both a distraction and inaccurate.
[Edit: I have just gone and had another watch of the video and the subtitles have disappeared 
though whether that was because I'd pressed the subtitles by accident or they were part of the upload initially I don't know] I can understand your English perfectly well without the subtitles. If you definitely want subtitles perhaps you could type them out yourself from your script?
As for what you speak about, yes I am sure you are right in what you say.
I myself do not think the cover obeys the brief.
It is clear that Dever wanted the forces of good on the right and evil on the left. I do not know why they were reversed.
I do not associate Thirteenth Century Northern European armour with spiked helmets, like those in the picture. This undermines Markus Tan’s theory of them being from Kakush. The soldiers are meant to be European in look.
The evil knights in black spiky armour referred to in the brief will be drakkarim, as you said. They are not in the picture. Instead the artist has mixed them up with the ‘goblinoids’ [meant to be Giaks I presume] so we get a mixture of a Giak and a Drakkar in the foreground.
The horned beastman with morningstar flail and dagger. I don’t know what that is meant to be.
I don’t know where Joe Dever got the eagle and heart design from. In the absence of any evidence to the contrary we can speculate that they are as you say, mercenaries, or Firalond's forces based on the colour blue. It intrigues me because he went to the trouble of attaching a precise image of the eagle and heart in his briefing yet we do not know who they are.
The skeletal monks in red robes are vordaks, as you say.
The blue-skinned warrior in the picture must be meant to be an ogron. It doesn’t tally with my idea of an ogron though. There is nothing ‘giant’ about him - he’s the size of a human being. I always imagined ogrons as blue (citadel miniataure style) ogres or maybe over-sized blue orcs.
From what the brief says (and especially what we see in the ‘rough page pagination’ sketch) I would say that Joe Dever wanted a depiction of two large armies charging towards each other, but not yet fully engaged, and not obscuring the fortress. To me it reads like he wants dozens and dozens of figures, not the ten or so we get in the picture.
though whether that was because I'd pressed the subtitles by accident or they were part of the upload initially I don't know]
