If you defederate from porn because of legal reasons you might as well be a private instance. Porn is allowed in every country bar a few on earth, and users even have the ability to turn off all NSFW content via their profile. Adult content is the easiest to solve for.
But that is just porn and I understand your point. At the end of the day, any instance is allowed to connect with whomever they want. Regardless of reasoning.
We had a few communities on Reddthat that were blocked on an instance level by hacking the database. After it subscribes between two instances you remove the db connection and it tricks Lemmy and doesn’t re-subscribe.
Within a month, third party applications will have multi account auto creation features with syncing of community subscriptions or abilities to connect directly into the communities that are siloed.