Since you are a lawyer why don't you define legal possession and how that concept can be applied to bitcoin for starters.
It is certainly subject to debate. My argument would be that possession of the privacy key constitutes the ownership and right to access the value denoted in the blockchain ledger. That is the basic premise of Bitcoin, correct?
This method is negated by any wallet stealer since both the thief and victim would have the private key. Given that info, how do you prove that you were ever the lawful owner of the BTC at that time? Only thing I can think of is maybe a receipt of some kind (exchange logs?) and that doesn't take into account the fact that the BTC could have been traded willingly off network by sending the private key to the buyer (or thief [I can know as a 3rd party{you can't either}]).