Sounds like a good time to involve the cops possibly? Sounds illegal to me...? They'll be able to link the protonmail and gmail accounts back pretty easily I assume
If there's some kind of staff site/portal/whatever with authentication, send them a unique link and somehow trick them into clicking it?
It's very much illegal to take the parent emails. The authorities have been contacted, but that route takes time. The person is sending a few emails a week now. From the content we have narrowed it down a handful of people, but there's nothing solid that we can take steps with.
If there's some kind of staff site/portal/whatever with authentication, send them a unique link and somehow trick them into clicking it?