When events refer to people (or other events for that matter) without a recommended_relay_url and which are not present on the relay which served the referring event.... then you might be kind of screwed. I've been battling this myself. I keep event-relay and person-relay associations in a database based on everything the client has ever seen, and look up those associations and hope. But sometimes there are none to use (especially when starting off). I really appreciate clients that fill in the recommended_relay_url field. So far I haven't fallen back to the "configure read relays" fallback solution, but that does work in today's nostr environment better than you might think.