I think if one were to write a guide to Identifiers and Identity for the Fediverse, one should also include the following
- Signing A method to show that the owner of the Identifier really composed the object. See fep-8b32
- Finding (?) The problem of given an identifier find the data associated with it.
Not sure about the word Finding.
I would consider Signing a solved problem with technical details left to be resolved. For Signing to work, one should use cryptographic identifiers, i.e. a public key as identifier and only the owner having access to the associated secret i.e. private key.
The question of Finding is harder, and probably does not have a simple answer. For the commonly used urls, the answer is DNS (which is centralized).