Phyrexian Unlife and Necropotence
I you are at 1 life you can only activate Necropotence once, this is because paying the life is a cost and that happens when you activate the ability, before anyone gets priority again [601.2]. So by the time you have the ability to activate it a second time you are already at 0 life, and thus unable to pay more.

As for Death's Shadow it does indeed get +X/+X when you are at negative life totals. From the Gatherer rulings:


  If your life total is less than 0 and an effect (such as the one from an opponent's Abyssal Persecutor) is keeping you from losing the game, Death's Shadow's ability will actually increase its power and toughness. For example, if your life total is -2, Death's Shadow gets +2/+2.