Paladin of Atonement’s first ability cares only whether you lost life last turn, even if Paladin of Atonement wasn’t on the battlefield when that happened. It doesn’t care how much you lost, whether you also gained life, or even whether you gained more life than you lost.

To determine how much life you gain for the last ability, use Paladin of Atonement’s toughness as it last existed on the battlefield. If its toughness was less than 0, you won’t gain life. (You also won’t lose life.).