It seems over thought. Blockchain is essentially a ledger (don’t even worry about the distributed part), and the bitcoin associated with a public address (wallet) is accessible to the holder(s) of the private key of the address. In my mind there is digital access to a digital account, while in theory the account is immutable and transactions on the ledger are immutable (ie no need to worry about double spend in this system) access can be lost or stolen.

That’s it! Ownership, like its suggested by the lawyer...well like all things in law: “it depends” and what it depends on are the facts as applied to ownership laws of local jurisdiction