Ownership is a legal concept, and thus it's up to the legal system to decide if "the person who figures out the hash collision is the owner", or if in certain cases it's not.
Yes, it may be hard to enforce a judgement, especially if the other party can't be identified or located. However, if the other party is reachable, then the courts can certainly force that person to, for example, reverse the transactions, or compensate the rightful owner (as determined by courts, not the algorithms), or have their stuff and liberty taken away by angry armed men.
The process was designed so that the person who figures out the hash collision has full de-facto control, but the designers of a cryptocurrency have no say in what the rules for legal ownership are, that's up to the legislators - and just as for many existing (including physical things) having full control does not imply ownership, i.e. the right to freely act with that thing without it being forcibly taken away, and the right to have your property protected by the state if someone (including someone who has the key) takes your property.
The users of a cryptocurrency are able to ignore the 'legal ownership' rules, though - they implicitly opt-in by using the system.
This is the key distinction between actually using _Bitcoin_ (i.e. running a full node and creating transactions), and using custodial Bitcoin (outsourcing it all to a third party using legal constructs).
If you're saying that the users of the cryptocurrency can't opt-out of the legal system - sure, no-one can opt out against men with guns (in practice what ends up happening is everyone hiding under pseudonyms and introducing a whole bunch of friction in an attempt to do it anyway).
I suppose my comment is intended to illustrate that this is all a sort of shell game played by non-participants.
Essentially, lawyers trying to glue themselves onto and extract consulting fees or whatever from a system which does not require them.
Repeated again for the sake of clarity - if men with guns ultimately decide transactions on the blockchain, the entire system is pointless. We can just delete it. We don't need the horrendously expensive resolution mechanism of PoW in that case, because there's already a resolution mechanism, that of men with guns.
> The construction was specifically designed such that the person who figures out the hash collision is the owner... It's not a matter for courts to decide
Anything is a matter for the courts to decide. Look up Popov v. Hayashi, where the courts tried to sort out who owned the ball that was Barry Bonds' 73rd home run. 2 guys caught it at more or less the same time. There was a dispute. The courts applied equitable principles to determine how ownership should be divided. The day will come when someone needs to make a similar call in relation to BTC.
In the specific situation of petertodd's transaction, the blockchain will resolve the first solution as the owner.
There is actually a technical flaw in those transactions in that a miner that is aware would just rewrite the receive address to their own as there's no signature.
But ultimately a confirmation will hit the chain and transfer ownership of the UTXO.
It's already a solved problem. This is literally the entire point of Bitcoin - decentralized consensus without need for trusted third parties.
Courts cannot move Bitcoin from address A to B. They simply do not have the power. They can muscle in and lock people in boxes and stuff but the actual coins cannot be seized because of the nature of the system.
Sure, courts cannot freely move Bitcoin from address A to B, but that's not really relevant. The relevant part of ownership in this context is whether moving Bitcoin from address A to B was legal, i.e. did you have the ("ownership") right to act with that Bitcoin? And the answer sometimes may be "no", even if you had the key (which you obviously did, if you made the transaction); the question "whether that transaction was fraudulent, did you commit a crime by making it" depends on who legally owned these Bitcoin, not on who knew the keys.
And in many cases courts can move Bitcoin, and have done so - hardware can be seized, people forced to reveal passwords, and Bitcoin transactions executed. For example, see the multiple auctions of seized Bitcoin by U.S. Marshals Service. They can't be guaranteed to succeed, but that's nothing new (it's not like stolen goods or cash always get recovered), and they certainly can try.
If someone steals something edible and eats it, courts also can't return that thing back to the owner, it doesn't mean that edible things are owned by whoever eats them. In both such cases, the courts can order (and, as much as possible, force) the culprit to compensate the rightful owner - but it matters who the rightful owner (in the opinion of the court) is, so it is meaningful to debate who owned it.
I think they do. Ownership can for instance be agreed upon by regular contracts, completely orthogonal to who controls some keys. (Your accountant might, for instance.)
The construction was specifically designed such that the person who figures out the hash collision is the owner.
The courts having rules to decide whom the coins belong is meaningless in that context.
Who owns a Barbie doll? Is it Mummy, Daddy or the child? It's not a matter for courts to decide because the question is meaningless.