The provisions of this article are cumulative with existing law and shall not be construed to modify an individual’s right to consent or refuse to consent to medical treatment if he is capable of making an informed decision, or to alter or limit the authority that otherwise exists under the common law, statutes or regulations of the Commonwealth (i) of a health care provider to provide health care; or (ii) of a person’s agent, guardian or other legally authorized representative to make decisions on behalf of a person who is incapable of making an informed decision. The provisions of this article shall not impair any existing rights or responsibilities which a health care provider, a patient, including a minor or incapacitated patient, or a patient’s family may have in regard to the providing, continuing, withholding or withdrawal of life-prolonging medical procedures under the common law or statutes of the Commonwealth; however, this section shall not be construed to authorize violations of § 54.1-2990.