It's not clear what the purpose of Provenance.agent.relatedAgent is. First, the full definition is just a duplication of the short definition. Second, there's no indication as to which two agents a relationship is being asserted to given that there are multiple agents being listed. Third, it's not clear what the relevance of asserting a relationship between agents has when recording Provenance. Fourth, if it is occasionall reference that the practitioner who performed some record change was the backup for some other practitioner, it's not clear that capturing this is in the 80% for Provenance.

So - figure out and document what the use-case is for relatedAgent - and ensure that it makes sense to capture in the context of a single Provenance instance - i.e. that it's relevant to the creation/update or deletion of record. If relevant, determine if it's the sort of thing that 80%+ of systems that capture Provenance information would record and if not, make it an extension. If it is core, document it properly and make it clear which agent is the "actor" - i.e. has the role - and which agent is being captured because of their relationship with the actor.