why does a multi-reddit need multiple instances to collaborate to create the feed?
by “create the feed”, I assume you mean “provide posts” when API call post/list is called?
content is replicated in all federated instances. You only need to use the local copy and merge all the communities of the multi-reddit.
Yes, that is what MultiPass would do, query the local PostgreSQL database. Right now Lemmy only allows this for a single Subscribe/Follow list per user… you have to create 3 different logins if you want 3 different lists of communities. For example, a “games” list, “music” list, “news” list… Plus, the current design does not accommodate logged-out users, they have no way to list multiple communities (other than “All”, local or merged remote+local).