Replace word "bitcoins" by "potatoes" and any judge will figure out on the spot what to do.
Potatoes aren't a digital construct thinly traded only on unregulated exchanges. I do agree that Bitcoin will need to be regulated eventually. It simply can't co-exist with fiat currencies without definition. However that day isn't today.
Of course. However, potatoes have value, they can be stolen too. Imagine a commodity exchange where you can deposit bags of potatoes that you and other customers have "farmed". Those potatoes can be sent to the exchange as well as fiat money (legal tender btw). Someone have stolen all the potatoes, exchange goes BK... Effectively a judge has only two choices:
1. Distribute all fiat back to depositors and leave potato sellers to hold the bag (an empty potato bag no less).
2. Value all the lost potato deposits in fiat, distribute whatever fiat left proportionally.
I bet it will be 2.