

I don’t think the ToS approach would be invalidated here via your Safe Harbor fork theory.
The ToS could state something like “you give us a worldwide perpetual right to use your content in any way we want including granting this right to whom we designate”
You still own your content but by having an account you agree to the ToS that lets them do what they want.
They just host it and are safe.
It’s a hard problem in the fediverse. It makes for a ticking time bomb of an issue. Imagine I am on a “everything is your own, we don’t sell your stuff” instance while another instance just copy pasted metas ToS. By posting a response to my instance, which then in turn is federated to the meta style instance I create something hard to solve. I can foresee other issues too.
I see your point. I just think it’s a difficult problem.