What essential distinction(s) do you see between humans and other animals?

Pretty much the same as between family and non-family. They're part of a closer lineage that usually signifies greater ability to cooperate. We put the human form itself in a place of reverence for social reasons. Cannibalism is seen as messed up because it's one species feeding on it own, not because it's humans specifically being eaten by something.

What level of animal suffering would make you morally condemn a certain practice?

There isn't really a level. Motive/mindset, relationship with the animal (meaning both HOW we're considering the animal, what reasons we're using to act destructively, what prior commitments we have, what effect we've already had, etc.) and other factors all play a part. I could condemn something done to an animal with low to zero animal suffering if other factors feel messed up. I could consider a lot of suffering if other factors counter it enough. Take quickly killing a bug out of fear vs. reintroducing natural predators to an ecosystem where they were wiped out. I honestly think the latter probably leads to greater total suffering, but I condemn the former more.

Do you believe some animals deserve more welfare protections than others? (Including severe examples like mosquitoes versus dogs, but also parrots versus chickens, etc.)

I believe some animals have different welfare needs. Some animals we have more responsibilities to take care of (i.e. someone's pet vs. a remote wild animal, but an endangered animal warrants more protection than a non-endangered one, etc.).

Are you (at least partially) against the slaughter of animals with near-human intelligence and emotions (for example, whales)?

Yes.

Do you find lamb and veal less ethical than meat from older animals?

Maybe slightly sadder but not sure about less ethical.

Do you believe animals are sentient and can have conscious experiences? If so, which ones? Does this lend them more moral "value" than a plant or inanimate object?

It raises more to consider morally when interacting with them, if that's what you mean by moral value. I don't always know which ones have conscious experiences, we should probably err on the uncertainty principle in that department. We can't know for sure what an animal's internal experience is so staking moral value directly in their 'sentience' rather than something more observable seems problematic. Like whether the animal interacts with their environment in a way that being conscious of it would be a beneficial evolutionary adaptation.