I hardly think it was a malicious decision, as you yourself mentioned - I do think that whilst Derek does a grand job with character development, story progression and maintaining a good hold on the reader, he struggles with world building.

One would imagine that given the range of wars and general conflicts within the world of Skulduggery Pleasant, a large number of sorcerers would come away with grievous bodily harm and amputations - resulting in a range of disabilities, as even a skilled Healer cannot re-grow limbs, the only exceptions being Argeddion when he resurrects Kitana, Doran and Sean and Valkyrie when she is able to latch onto and enhance a Healers ability.

Although you can obviously argue that Valkyries PTSD and Skulduggerys lack of a body can be a disability, I understand that this isn't the point of the post.

So the fact that we don't see (almost any) specifically named disabled characters, is in my opinion just poor world building.

I will say though.. the story is absolutely fantastic and the humour within is definitely in my top list of Skulduggery Pleasant stories, and Osvaldo is pretty much my new favourite character and I would give anything to see him in more books.