#life total

[magicjudge]

If I am at negative life but not dead due to Angel's Grace/Phyrexian Unlife etc. would a Death's Shadow I control gain P/T via double negative?

Yes.

Giving a creature a buff or debuff to its power and/or toughness is one of the few places where magic can make use of negative numbers, and Death’s Shadow benefits from you having a negative life total if you can stay alive.

For example, if you’re at -10 life, your Death’s Shadow will get -(-10)/-(-10) = +10/+10 from its ability and be 23/23.

[magicjudge]

In a 2HG game, one team controls Ayli, Eternal Pilgrim, or some other card that depends on your life total being 10 or more life above the starting amount. Is this when the team is at start+10 (normally 40), or is this counted per head having half the team's life total, thus requiring the team to be at 50+ life?

Starting life total in 2HG is 30, not 40.

Each player’s life total is the team’s life total, so you can use Ayli’s ability if your team is at 40 or more life.