So, full disclaimer: I mostly rewrite rules over making them myself. Where I do create content, I do so by copying stuff that's similar and tweaking it in a kind of simple "zero-sum" fashion. I do this because my understanding of the intricacies of BFG balance is somewhat limited, so anything I have to say on these rules should be taken within the context of comparing it to the closest thing I'm aware of.
...and that would be the Desecrator Battleship.

Right off the bat, I notice that the Dethroner has 5 turrets instead of the traditional 4 for Chaos Battleships. I'm not sure the extra turret is necessary unless there's a very specific reason for it to have all 5.
Suggestion: 4 Turrets and drop the price 5pts.So if we shave off those 5pts and the turret, we have a vessel which is 35pts more expensive than a Desecrator. In terms of direct comparison, we're now looking at:
- -3 Torpedo Strength
- +5 Battery Firepower (total)
- +2 AC capacity
- -2 Lance Strength
- -2D6 on AAF
- Option for +5cm Speed by losing 2 AC capacity
All the ranges, arcs, and ACs are the same, so the above list is really what we need to evaluate. For 35 points, there's not a lot of benefits to using the Dethroner over a Desecrator. You're gaining a little bit of gunnery firepower and a couple of AC squadrons, but you give up lance strength, AAF speed, and torpedo strength. Moreover, even if the weapons are brought up to parity for the +35pt cost, now we're left with a slightly toothier Desecrator for a few more points...
At this point I'm not sure exactly what to do

. My gut says something like boost it to S9 torpedoes, and make both of the weapon batteries F10. If we do that, you're trading Lance Strength for AC Capacity, and gaining +11 Battery Firepower in exchange for 35pts and the weakened AAF drawback, which seems maybe reasonable? But again, it's still just a big slow Desecrator with a fatter top gun (that's resistant to crit damage, since it's 2 guns), and a pair of lances swapped for bays. While it might be worth its points then, I'm not certain that it's necessarily unique enough to really meet the requirements of a totally new class of Battleship.
That's about all I've got, I'm sorry to admit. Hopefully someone more knowledgeable than me will swoop in to explain how I'm all wrong

. I do really like the look of the ship and the fluff (I'm a sucker for that Repulsive class prow), so I hope you can get the balancing worked out.

Cheers,
Xca|iber