Wound Reflection's ability checks only whether life was lost. It doesn't care whether life was also gained. For example, if an opponent lost 4 life and gained 6 life during the turn, that player will have a higher life total than they started the turn with but Wound Reflection's ability will still cause that player to lose 4 life.

If Wound Reflection's ability resolves, then an opponent loses life later in the same turn, that life loss is never counted by Wound Reflection.

When Wound Reflection's ability resolves, it checks how much life each opponent lost over the course of the turn, then it causes that opponent to lose that much life. It doesn't matter how the opponent lost life or who caused it. It also doesn't matter if Wound Reflection wasn't on the battlefield at the time some or all of the life was lost.

Multiple Wound Reflections each trigger separately. For example, if an opponent lost 1 life, the first one's triggered ability to resolve causes them to lose another 1 life. Since they've now lost 2 life, the next triggered ability causes them to lose 2 life, and the next 4, and so on.