> Cats can vary wildly. One of my cats seems dumb as a box of rocks and haven't even grasped the idea of object permanence

Similarly I've seen cats have one of two reactions to a mirror: ignoring it entirely or actually using it by e.g. looking me in the eyes and meowing at me through it. While I've not witnessed it personally on the internet there's also tons of videos of cats freaking out and trying to fight the other cat in the mirror.

This supports the idea that the gamut of intelligence in cats is quite wide.

Our dog remembers the location of toys at the park over long periods of time, though being able to sniff them out probably helps. He also expresses genuine surprise and suspicion when he sees novel objects (e.g. the large Christmas tree that was put up in the park, a horse and rider), because he knows they're not usually there. He doesn't like fat people, which is embarrassing, but I also knew a dog as a teenager that freaked out anytime it saw someone who wasn't Asian. Just given the amount of back and forth communication that happens between most owners and their dogs, they're very clever. Cats are some of the best hunters in the animal kingdom, but I've never felt that they're there in the way that dogs are.

I've known many dogs that fail this test, too.

>Does the scent gland do anything more than just stinking?

It's part of their communication system. There's no direct corollary in human qualia, but you might say it's akin to permanently destroying your ability to flirt or tell other people that something belongs to you. You would still experience the impulse, but not have the cognitive equipment to do so any longer. Removing scent glands destroys the physiological equipment, of course.

I mean, you're removing part of a living animal for human convenience. If the ethical issue isn't obvious I don't know what to tell you.

The practice has been banned in the UK for almost 20 years, under the exact same laws as ban declawing cats. It's unnecessary mutilation with no medical justification.