Sustaining Spirit won’t prevent you from losing the game if your life total is 0 or less or some other effect causes you to lose the game.

The second ability is not a prevention effect. It stops unpreventable damage from reducing your life total below 1.

The ability doesn’t change how much damage is dealt; it just changes how much life that damage makes you lose. An effect such as Spirit Link will see the full amount of damage being dealt.

If an effect asks you to pay life, you can’t pay more life than you have.

The second ability applies only if your life total is being reduced by damage. Other effects or costs (such as “lose 1 life” or “pay 1 life”) can reduce your life total below 1 as normal.

Paying cumulative upkeep is always optional. If it’s not paid, the permanent with cumulative upkeep is sacrificed. Partial payments of the total cumulative upkeep cost can’t be made. For example, if a permanent with “cumulative upkeep {1}” has three age counters on it when its cumulative upkeep ability triggers, it gets another age counter and then its controller chooses to either pay {4} or sacrifice the permanent.

Does not affect damage if you are already at zero or negative life. You still take it all.