I had to google the Dead Sea Effect: it means that when a workplace has a bad culture, the most skilled and ambitious workers are first to leave and those who have a habit to do the minimum will remain.
Anyway, I'd say this effect is weaker in this situation, because many workers are there truly to serve their country, not merely as "generic workers". They might feel that they have something important to contribute to the society, which means there's an actual chance for even some of the best to return.
Each threadiverse community is visible as a single user in Mastodon. All the comments come into one pile, not organized into clear threads. It's a very rudimentary support. But yeah, you can join a threadiverse conversation from Mastodon, but you cannot share any Mastodon content in threadiverse.
From within Lemmy, there is zero possibility to utter anything on Mastodon, except as a reply to a message a Mastodon user has made in a threadiverse community.
It's actually very easy checking how this conversation looks from within Mastodon! Just go to any Mastodon instance (such as http://mastodontti.fi/ ) with your web browser, click the search tool and search for a "user" called @fediverse@piefed.social .