Pikuach nefesh is doicha Shabbos, it certanly should be doicha dina dimalchusa.
Not when the person acquiring the organ is selling it for profit and not out of the goodness of his heart to help save a life. If one needs marijuana for medical use (glaucoma, chemotherapy side effects), it should be made available. But the law of the land says it is illegal to use or sell it for use, and if a rabbi is caught selling it to people who need it, he will still be arrested for drug peddling. The law should be legally circumvented, but organs should not be bought and sold on a black market. The chillul Hashem that this caused cannot be undone.