Why doesn't pain matter? It's almost the canonical example of a valenced state that practically any moral theory is tasked with making sense of on moral terms.
Plants most definitely do not have different but equivalent mechanisms. They don't have subjectivity. Moral theory is about, among other things, explaining the meaning of moral behavior and language as it manifests in real people, and well-being is just as real as health. At least it is on moral realism which is a perfectly mainstream view in moral philosophy.
You are not making any sense, and literally won't understand if you never got beat-up physically in life, perhaps as a kid, and preferably more than once. One has to experience pain first-hand to empathize with it. You must have lived an extremely sheltered life for this to not happen to you, and so you don't understand. Your nihilism doesn't relate with people.
Yes, plants react functionally to damage, but in no way has any shred of consciousness been demonstrated in plants. You are just out to seek any excuse necessary to kill animals and perhaps humans too, with no difference.
You are not making any sense. Perhaps consciousness hasn't been demonstrated in plants because we don't have the correct definition of consciousness or are looking for the wrong things. Why do you have this arbitrary bias towards animals? Perhaps it's time to re-examine your fundamental assumptions.
Moral theory is bullshit. It's just made up.