It fails to bring the creature back if the creature is not still in the graveyard at the end of turn.

It works for all players and has errata to remove the "your graveyard" text.

The creature comes back even if Lifeline leaves the battlefield after triggering, but before it resolves.

If multiple creatures are coming back, they come back one at a time, not all at once. This is because Lifeline triggered once for each creature and set up a separate "at end of turn" effect for each.

If more than one creature is on the battlefield and all the creatures on the battlefield go to the graveyard at once, then none of them are returned at end of turn. This is because Lifeline's ability has an intervening-if clause, which means that there must be at least one creature on the battlefield at the time the ability resolves.