I don't really get what you mean by ''passive of adversity''. Can you be more detailed?
What is the object of 乗り移られちゃう? If ラルヴァ is the agent, wouldn't it be ラルヴァに乗り移られちゃうじゃない?
indirect passive/passive of adversity
(昔のエルンスト人がラルヴァに)誰かに乗り移られる
昔のエルンスト人: subject/the one who is adversely affected by the event
ラルヴァ: agent
誰か: target