Why is that? It would require any implementation that wants to provide metadata (about FEP support, for example) to have an instance-level actor with additional inbox/outbox and other actor-related requirements. It seems like an unnecessary coupling of independent features.
Maybe I could be convinced otherwise, but I currently think it would not be a good idea to consider the instance-level actor as a proxy for the instance itself. The notion of an instance-level actor is still a bit vague. How is it technically different than any other actor? Why can there only be one? Maybe an instance wants an “instance actor” representing the moderation team, and another for technical site administration, and another for relay support. (Those would probably be Group or Organization actors rather than Application or Service.)
As far as I can tell, an “instance actor” is any actor that is referenced by the instance metadata with the instance actor link relation. There might be different authorizations for an instance actor compared to a non-instance actor, but that hasn’t been specified yet. In the examples of instance actors I gave above, each of them might have different authorizations, depending on their role.