So I have a 5 color deck and I was wondering how Necropotence and Phyrexian Unlife interact in certain circumstances. I understand that once my life becomes 0 or less I cannot pay life to draw cards, but if I were to have 1 life left, would I only be able to pay one life or would I be able to pay more than that and draw more cards dropping my life total into the negatives, like an attack when you have 1 life left for 5 damage would leave you with -4 life and 0 poison counters. The way I currently see it is I would only be able to pay 1 life exile a card then my life total is 0 and I can no longer exile anymore cards. Normally in this deck I'd be using Necropotence with Venser's Journal and Elixir of Immortality,but I was wondering if Necropotence might benefit from Phyrexian Unlife as well.
Also somewhat of an aside to this question but I also use Death's Shadow in this deck, am I correct in assuming that if my life total is -4 death's shadow would get +4/+4 making it a 17/17.