Begginer question about lifelink and Evra
Lifelink on a creature causes you to gain life whenever that creature deals damage in any way. In combat, unblocked creatures deal damage to the defending player, and blocking creatures and blocked creatures deal damage to each other. So if Evra is in any of those situations then it will deal damage and you will gain that much life. One common exception is if Evra blocks or becomes blocked, then the other creature is removed from combat before the damage is dealt. Then Evra will have nothing to deal damage to, so it won't deal any damage and you won't gain any life.

Regarding Evra's other ability, you are correct that if Evra's power somehow becomes 0 before the ability resolves, the ability will make your life total 0 and you will lose.