@grishka to your other questions: "where does an object that doesn't make sense outside a collection go?" I would reply that you shouldn't have such an object. Each object should have a unique IRI that can be referred by, independent of the collection (or collections) that it is a part of.
That's why in FedBOX I have generic read-only collections for objects: https://federated.id/objects
That collection is (I imagine) the same thing as you describe in the FEP.
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@grishka
The only objects that can't be referenced through that /objects collection are the ones that are not addressed to either the shared inbox of the server, or the ActivityPub public namespace. (ie, objects generated by private Activities - addressed only to specific actors' inboxes)
I'm still not 100% sure this is the way I want to keep things, but for now it serves me well, and lowers the complexity of managing objects' IRIs.