We don't have canon examples of a lot of things.

But we have an explicit canon statement by a recognised authority in the spell school in question. That should trump guesses based on inferred similarities between different instances of different spells.

Which is more likely, that the Vanished animals follow the trends already observed for all the related magics, or that Rowling makes an exception for the animals and Hogwarts is a charnelhouse?

Given that

1) the trends you cite are only there if we assume that every instance of something disappearing uses identical magical mechanisms to the Vanishing Charm

and

2) Rowling appears to have no conception of non-sentients' rights whatsoever (cf. Transfiguring hedgehogs into pincushions, some of which still quiver in fear when faced with pins)

I believe the balance of evidence favours the "charnelhouse" claim. To clarify, I don't believe that Rowling makes an exception for the animals: whatever magical effects apply elsewhere, the specific spell known as the Vanishing Charm is intended to make its target enter "non-being" and permanently disappear.