Depends on the uses. Food Theory did a great video about this very thing, covering preferred taste, consistency, price, protein / fat content, and bake-ability: https://youtu.be/df8FRfVtVNw
Lactose is simply the kind of sugar/ starch in the milk.
Technically yes. But of course they would (and can't really) do that. But you could also eat stuff like roadkill and it's vegan. Veganism as a moral philosophy has nothing to do with food, it's about respecting and granting animals the same rights as humans (as far as applicable, not stuff like voting).
Only if you define vegan as to strictly avoid any animal product (and define humans as animals). A somewhat looser Definition says to avoid animal exploitation.
So a product made by a non-domesticised animal in a natural way - e.g. Penguin guano - could be seen as vegan. The animal produces it anyway and the product isn't won through keeping the animal captive and / or "stealing" from it.
After all I wouldn't be too strict with definitions here.
Most honey wouldn't be vegan but perhaps an abandoned hive could be harvested. Or infertile eggs from an abandoned nest? Bits of sheep's wool collected from a spiky bush?
Yeah sure. Maybe you could make the argument that humans should leave stuff like that for other scavengers who need the nutrients to survive, and instead opt for plant foods. But at those edge scenarios you would then also have to take into account the impact that plant agriculture has on wildlife. It's quite possible that scavenging and gathering is the most vegan option, but seeing how it's neither viable for a lot of people nor something that often comes up in daily life, it's easier to just generalize vegan food as plant based.