I was under the impression limited liability was exactly why corporations are legal persons. So they can sign contracts and be liable for violations of those contracts as well as breaches of law.
Well -- perhaps it's a person in some senses -- but it's not a citizen.
Not yet.
Yes, but, because people own corporations, and want to use the corporation can use the corporation to exercise their rights, it is frequently convenient to treat the corporation as a person when deciding how to apply the law to corporations. :P