I'm personally of the mind that we should be imagining a world where all 3 of these solutions are at play. 1 is absolutely the most important, and Admins should be taking an active role here where possible (particularly as it relates to dead community cleanup). I personally think they are the missing element needed to negotiate these sorts of consolidations. 2 and 3 on the other hand are pretty simple features and even if Lemmy never takes it on, I think it's reasonable that any one of the new fediverse link aggregators could take this up. The only other thing I'll say is multi-communities absolutely must be sharable. Ideally, it should even be possible to link multi-communities with the "!" syntax or similar.
I've heard that bluesky is attempting something like this but I'm dubious about bluesky generally. Otherwise, I think this is a killer feature of the fediverse that has really not yet materialized.
It could probably be done with a dedicated local client