I think it's all insane. A collection of everything related to a thread is a collection is a collection. Distinguishing between those that can contain an implied Create and/or full activities and/or a URL and those who can't/shouldn't seems like a nightmare of unnecessary complexity.

An item in a collection can be a number of things (URLs, objects, activities), any of which you need to be able to accept and process to be considered ActivityPub/ActivityStreams compliant at a basic level.

I don't believe we really need one collection for the posts people and another for the conversation people containing different representations of the same content - but here we are. Perhaps we need yet another representation for the URL-only folks? If you think that's madness, then why isn't this madness?

We normally do full activities, but after some prodding, I've begun implemented the forth-coming FEP about context vs contextHistory. So now the default context is objects for Create|Update|Delete Note|Article and activities for everything else, and contextHistory is full activities for everything associated with the opening post-of the conversation; but what a nightmare...  

We do not typically provide URLs as collection members, because you may need a signed activity to access and validate third-party objects which have source access control enabled.