(a) Nothing in this chapter shall impair or supersede any legal right or legal responsibility which any person may have to effect the withholding or withdrawal of life-sustaining procedures in any lawful manner. In such respect the provisions of this chapter are cumulative.

(b) Nothing in this chapter shall be construed to condone, authorize, or approve mercy killing or to permit any affirmative or deliberate act or omission to end life other than to permit the process of dying as provided in this chapter. Furthermore, nothing in this chapter shall be construed to condone, authorize, or approve abortion.

(c) This chapter shall create no presumption concerning the intention of an individual who has not executed a declaration to consent to the use or withholding of life-sustaining procedures in the event of a terminal condition, a coma, or a persistent vegetative state.

(d) Unless otherwise specifically provided in a durable power of attorney for health care, a declaration under this chapter is ineffective and inoperative as long as there is an agent available to serve pursuant to a durable power of attorney executed in accordance with the provisions of Chapter 36 of this title, the "Durable Power of Attorney for Health Care Act," which grants the agent authority with respect to the withdrawal or withholding of life-sustaining or death-delaying treatment under the same circumstances as those covered by a declaration under this chapter.