Write this down in the record books: Psion defends a Mongoose product against a Bramadan attack!
I should preface this by saying that I, too, have serious balance concerns with Chaos Magic. However:
"Together with the significant drop in quality and quantity of "grey box" fiction"
I am astonished that you see that as a bad thing. You read fiction once and you are done with it. Mongoose went way overboard on fiction in earlier products, and I am glad to see they are cutting back. There is a place for some flavor text, but if I wanted fiction, I would buy a fantasy novel. This is a
gaming product.
"With atmosphere diminished from Demonology and Necromancy the weight of the book falls on the game mechanics of chaos magic."
Atmosphere? When you are playing the game it is on the DM to create the atmosphere. Again these are
gaming prouducts. Mechanics should
always be a consideration. And I see
Demonology as being on at least as shaky ground mechanically than this one.
" It also means that chaos mages have pre-assigned highest stats: Cha and Con (for hit points) - annulling another great achievement of d20, that of making various stat assignments valid options within a single class."
Huh? Do you regularly play wizards without intelligence as one of their highest scores? OR monks without their highest three scores in wis, dex, and str? I see nothing special here.
"beyond the obvious emphasis it puts on already extremely useful stat: Con, it opens the entire issue of healing. The Player's Handbook states that any amount of magical healing will whip out equivalent amount of the subdual damage the character has suffered making healing chaos mages significantly easier."
If the chaos mage takes subdual and real damage in the same encounter. One of the other, you are hosed. That said, that's hardly a worthy detraction because your default case is that you don't need healing at all.
"Furthermore, even with decent clerical support chaos mage is a veritable damage dealing machine"
After you previous paragraph, I find it odd you consider clerical support a non-issue. I agree with you inasmuch as I don't like this kind of advantage/disadvantage tradeoff. However, keep in mind that this "clerical support" is not so easily given; you are draining the party of its healing resources. Further, you also seem to forget that the HP drain on the chaos mage puts the chaos mage at a disadvantage
during combat. Accumulation of damage before the cleric gets there puts the chaos mage that much closer to the brink, and the closer you get to that one magic missile or arrow that will lose the party their artillery.
" This on the other hand reduces the versatility of the Chaos Mage in comparison with the mage seriously damaging the very reson d’etre of the class."
Frivolous != Flexible. There needs to be a balancing factor somewhere for the flexibility. This is it. It doesn't annul the flexibility.
" It is possible to house rule many of this rules to get the somewhat better system. Backlash chance can somehow be related to the level of a spell being cast in order to avoid “always cast the most powerful ones” problem."
You heard it here second, folks!

(i.e., I said this in my review.)
"And 3) A distinctly new flavour filling an important niche in fantasy gaming.
While I believe that 3) was true for Necromancy and Demonology I simply do not see it as a case for Chaos Magic."
??? What?!
Demonology and Necromancy are not new in flavor. They have lavished more detail on it perhaps, but I do not see them as necessarily distinctly new. Au contraire, it appears they have tried to reconstruct some rather classical tropes. Chaos magic is the less travelled road here, and easily deserves the title distinct.
"They are wading ever deeper into the babe-art territory but they are still on the right side of (my subjective) line of good taste."
They crossed mine with the boob-groping-skeleton in Necromancy and a boob-groping-chaos-monstrosity isn't any better.
Anyways, all in all I think Necromancy is the better balanced book, and agree that Choas Magic is on mechanical shaky ground. (But then, so it Demonology). But I think you dismiss the effect of damage on the chaos mage a little too easily.