Since some animals taste better than others, which one tastes best? Do hominids taste any good? Or maybe some predator which vampires might rarely encounter? Killer whales?

Carnivores taste better than herbivores; omnivores are presumably in between. I implied, although perhaps not strongly enough, that Edward tried a shark; it was more or less in line with predators in general. There's not enough variance to expect there to be a particularly excellent species somewhere. Individual vampires have preferences within animalspace but there's no way to predict what Bella will like best until she's sampled a few kinds.

Are there any ways to process/cook/spice animal blood so that it tastes less bad?

Nope. It's best straight out of the jugular.

Is there a way to synthesize a yummy blood substitute? This would be an ambitious research/engineering project but with immortality, you might as well get started.

Maybe. Bella's probably going to start something along these lines once she can stand being around human blood - she wants to figure out what there is in human blood that makes it tasty. (Carlisle could start something like this, but while he's a fine character and whatnot, he does not have an experimental mindset, and would be unlikely to give up large chunks of his time to satisfy this interest of Bella's. For example, instead of thinking about what might cause vampire venom to cause turning, and then getting a lot of it directly into Edward's heart when he was administering his first turn, he settled for recreating the bites he himself had received. This, it turns out, made the process unnecessarily drawn out.)

Your other questions have, as you noted, been addressed.