You are conflating terms and misusing English. You even do it in the question itself when you say “possession of the seed phrase”.
You can understand that if you steal a car, you don’t own that...
Thank you very much for taking the time to write such a comprehensive answer. Your examples has made it clear to me that you indeed can't own bitcoin. If you can't own it, how can you take self-custody of it?
Maybe you can say that you have access to a specific set of UTXOs and nobody else has.
Maybe you can say that you have access to a specific set of UTXOs and nobody else has.