with all this talk about sharing models... the sale pitch for bim... i find that no one wants to share due to liability...thus unless you can find a legal way around your sharing of your model.... don't count on others wanting to take on that liability....i find bim useful strictly as an inhouse tool.... the give the paper to the rest of the world and let them do their job.
You might want to look into the Consensus Docs BIM addendum. Both of your questions would be answered on a contract by contract basis. My default answer to both questions would be "the author", if you want further explination, just let me know. The BIM addendum usually answers more questions than most people have even considered at the time they start asking.
The owner owns the model a it is virtual asset.
The designers of the model take the legal responsibility of the model, as would a contractor take the responsibility of the real asset.