i will, at any rate, not be including guns in the story even if it makes sense, because I really, really, really hate them and gosh darn it I am doing this because it's fun.

That sounds like a good reason to me.

and venom is the only thing that leaves a scar. (Thence Jasper's.)

Ahh, that was another question I had.

Edward claims in canon that he could kick out the wall of an out-of-control airplane and jump out without hurting himself, while carrying Bella, without getting her killed.

This sort of thing is why I like to assume that the 'magic' of vampire strength is not quite limited to what, say, super strong muscles would result in. A lightweight terminator couldn't pull off a human saving plane jump from hundreds of meters up. It would be physics applied to the human body that is the limiting factor rather than rescuer. But 'vampires can do that kind of thing' seems a perfectly reasonable explanation. I'll apply similar reasoning to vampire (and wolf) combat. All sorts of inconsistencies in the physics can be blurred over if they are just included as part of the "vampires + shapeshifters" counterfactual.