It's a big change for sure, although I am not able to understand why the early death of Murat would benefit Naples: the political savvy of Caroline would be available anyway. Murat's death opens the door to a longish regency, since Achilles would be just 13 and his brother 9. Worse than that, Achilles growing up without a father became very eccentric (some unkind person might say worse than that): growing up in Naples as the future king would certainly be much better than growing up in Austria with little money and under suspicion, but still he may not end up as a full functional adult. Anyway you're the author, and I suppose you'll explain the narrative reasons for this surprise.
As far as the Savoys are concerned, I do fully understand and share your distaste. There are however ways to change OTL succession, and spin the wheel: Vittorio Emanuele might die in September 1822 when his nursery was consumed by a fire, and in such a case the succession would go to Ferdinando, who was born in November 1822 (the death of his infant son might also send Carlo Alberto over the bend, since he was already depressed, and open the door to a very long regency, but maybe it would be too much of a feuilleton); another possibility is that Umberto I does not marry his bloody first cousin Margherita, who had a very bad influence over him: I suppose there is some other princess who might be chosen, and whoever she is it would be a major improvement.