do negative life totals count as 0 for all purposes except raising it back up
That's close, but not quite accurate. The actual rule is:
Quote:
107.1b Most of the time, the Magic game uses only positive numbers and zero. You can’t choose a negative number, deal negative damage, gain negative life, and so on. However, it’s possible for a game value, such as a creature’s power, to be less than zero. If a calculation or comparison needs to use a negative value, it does so. If a calculation that would determine the result of an effect yields a negative number, zero is used instead, unless that effect sets a player’s life total to a specific value, doubles a player’s life total, sets a creature’s power or toughness to a specific value, or otherwise modifies a creature’s power or toughness.
What that boils down to is that your actual life total is used in any necessary calculations or comparisons, even if it's negative. Then, if the result ends up being a negative number and you're trying to do something that doesn't handle negatives, you use 0. (And the only things in Magic that properly handle negatives are setting or modifying creature P/T and setting or doubling a player's life total.)
So
Beacon of Immortality on a player with -10 life will actually leave them with -20 life, for example, and
Repay in Kind can potentially set everyone's life total to a negative number. Going into negative life with
Death's Shadow on the table will actually
boost its P/T. And so on.