Two new monsters this morning! What a pleasant surprise. I've updated the first post with the new links and the updated leader-board status, and I'll get right in to the review.

Auvvik
Overall, a nice, low-CR monster. All your math checks out.
I was originally thinking of having them be monstrous vermin like monstrous spiders with a bunch of different size categories, 'cause there's this other myth regarding whale-sized caterpillars. I figure I can just apply 'giant' a few times and get the desired results, though.
I can easily imagine some kind of encounter with a witch and all her auvvik pets. She uses hold person or some other sort of paralytic magic, her pets drain the paralyzed victims.

Disembowler
I will say, I've certainly been looking forward to seeing this monster completed! It's one of those weird critters that really adds flavor to the setting.
I don't know about check result=save DC. The two scale very differently. This critter's average DC is 26, I don't think a 6th level character can save against that, short of a natural 20. I would change the save to a standard DC (10+1/2 HD +cha in this case). Granted, it takes away the value of the perform ranks a bit, but still.
Other than that, everything checks out! All the math is right.